I decided to flesh out the idea from here into a full IA entry. /u/Kelaador or anyone else is free to take this and use as they like.

Origins

The Ice-Blood were created in the Fourth Founding in mid-M32, shortly after the victory in the War of the Beast. As with a number of other chapters in that founding, their primary mandate was to protect a region of space from Orks in order to prevent another Beast from arising in the future. With this goal in mind, they were placed in the eastern edge of Segmentum Solar, and chose a homeworld which was strongly resistant to Orks; the Ice World of Coldhome, whose hardy miners they judged would make fine initiates, and whose freezing climate made it difficult for the fungoid Orks to get a foothold.

Why this chapter was chosen to be founded from the gene-line of Sanguinius is unknown. Certainly, Blood Angels successors are excellent at matching the Orks strength for strength in close combat, but in many other places Ultramarines or Imperial Fists were chosen instead, or even the ranged specialists of the Unforgiven. Regardless of the reason, they initially seemed much like the other chapters of their heritage; they suffered from the Red Thirst and the Black Rage, and were both beautiful and terrible of aspect. Their homeworld's culture, however, introduced some changes within the first two centuries. The spirit of endurance and stoicism that pervades Coldhome's culture affected their approach to their gene-seed flaws, and the spiritual practices led them to the "Angel's Ice" ritual, which sets their Death Company apart from the others.

Their war record has been exemplary but unassuming; they have put down several Ork WAAAGH!s in their early stages, and the sector Coldhome sits in is considered remarkably free of xenos interference. Rebellions and heresy are not notably common or uncommon; a revered Master of Sanctity of the Ice-Bloods, Chaplain Chugiaq, is known to have said "The rage of Sanguinius is a terrible thing, too destructive to unleash lightly even on the corrupted children of humanity, depraved as they might be". In line with this statement, they attack Orks and other Xenos incursions at first notice, but take a watch-and-wait approach to rebellion and heresy until they are satisfied that it is beyond recovery and requires astartes intervention.

Homeworld

The recruiting world of the Ice-Blood is the Ice World of Coldhome, a Mining World in eternal winter which is nonetheless fairly forgiving in its environment for a world of its type. Its airless, tidally-locked moon, called "Starwatcher" by Coldhomers, holds the fortress-monastery of the chapter on its far side. Coldhome is a dangerous but livable place; its great bears can shrug off any shot from a lasgun which isn't a direct hit, and its giant moleworms eat carrion and are not above crushing human mining caverns to create more of it. Despite these dangers and the warlike demands they place on the miners of Coldhome, their civilization is largely calm and peaceful, consisting of loosely-federated clans with an extensive network of family relationships tying each clan's family gods to those of their neighbors.
In this web of kindred gods, the Emperor and the Ice-Bloods stand apart. In Coldhome thought, the Emperor stands apart from the family, distant but paid fealty by all. The Ice-Bloods are the Emperor's clan, and like the clan gods owe Him obeisance, the clans of Coldhome owe the astartes their service and sons when asked. Coldhome shrines to the Emperor are not terribly common, and where they do appear universally show him as a winged man with pure white hair in gold armor trimmed with red. In this, they mix Sanguinius with his father, but the Ecclesiarchy sees no cause to object, since their reverence and devotion is impeccable.

Beliefs

The primary factor that sets the Ice-Blood apart from other Sons of Sanguinius is the manifestation of the Black Rage. While it seizes them during battle, as it does other chapters, they do not cloister their Enraged marines, but send them to a chamber near Coldhome's north pole, called "Angel's Ice" and untouched by any of the mortal inhabitants, where they conduct a ritual which has been passed down since the early days of the chapter. On a large dais of blue ice, which has never melted a centimeter lower despite centuries of ritual, the marine is chained, and their blood drawn with ceremonial knives made to cut so that healing is difficult even for Larraman cells. Those who have undergone the ritual and survived with sanity intact say that the hallucinations of the death of Sanguinius change in character as this blood drips out, first changing the great duel to appear set in a cave of ice, then growing slower and the ice closing between the two primarchs until the Rage fades. These sane survivors are rare, however; for every hundred marines who enter Angel's Ice, twenty die there, seventy or more lose all individual volition, and several of the remainder become deranged. All lose what psychic gifts they had, though perhaps one in a hundred of the few who keep their lives and their volition gain new psychic talents, an even smaller fraction of which had psychic gifts prior to the ritual. (Even in that case, their gifts change substantially in nature and aptitude.) Those who lose individuality are called the "Ice-Dead", and the trim of their armor is repainted ice-blue. They are automaton-like, responding to direct attacks and taking orders from the chapter Chaplains but otherwise doing little. The deranged "Ice-Mad" are given the Emperor's peace, and the few sane survivors, sometimes called "Ice-Born", are made officers for the Ice-Dead to supplement the chaplains. While this treatment is effective at salving the Black Rage, the implication that the Angel's Ice ritual entirely cuts off the marines from Sanguinius puts their cousin chapters ill at ease. It also is worse than useless at treating the Red Thirst; the Ice-Dead have no volitional control to hold themselves back from indulging in the Thirst, and without constant orders from a chaplain, they will succumb immediately and not resurface until there are no further foes to attack. For this reason, the Ice-Dead are deployed very much like the Death Company of other chapters.

Combat Doctrine

Like other Blood Angels successors, the Ice-Blood prescribed tactics are unremarkable and in compliance with the codex astartes. Like those others, their de facto tactics are substantially divergent, as the Red Thirst and Black Rage make even the most experienced veterans occasionally lose discipline and charge into melee. They display more control on this front than most descendants of the Ninth Legion, but remain susceptible. For this reason, they are disliked by Imperial Guard generals and other imperial commanders, who see little difference between them and their cousins. The exceptions are those in their sector, who have a higher opinion of them, though they stil display mixed feelings. For their part, the Ice-Blood do not mind, as it means they are rarely called to defend areas outside their sector.

Organization

The company and specialist structure of the Ice-Blood follow the standard mold of a Sons of Sanguinius chapter. Assault marines see somewhat heavier use than in a strictly codex-compliant chapter, special armaments optimized for close quarters combat are in use, and the Apothecary and Chaplain roles are altered into the Sanguinary Priest and steward of the Death Company.

The Death Company of the Ice-Blood is the largest standout from this pattern. Their Death Company is not characterized by holy fury as most are, but by difficult-to-control bloodlust and depersonalization, like very serious cases of the Red Thirst in other chapters. These still require skillful guidance for maximum effectiveness, but have much longer useful lifetime, fighting in dozens or, occasionally, hundreds of battles as members of the Ice-Dead. Accordingly, they are still considered to be active brothers of the chapter, counted towards the Codex-assigned thousand, split between the 1st Company veterans and the reserve assault company (with no fixed distribution). When the ranks of the Ice-Dead exceed the nominal 100 of a company, they are restructured into a company with Captain and other officers. To stay in the Codex-mandated size restriction, the veteran company and tactical and assault reserves are fielded under-strength at these times; brothers shift from the tactical reserve and into the veterans and assault reserve to maintain a proper balance of forces among those brothers with their full faculties.

Colours

The standard armor pattern of the chapter is a dark forest green with crimson trim. Sanguinary Priests wear the usual white and mark their chapter by painting their trim the chapter's forest green, and chaplains not assigned to shepherding the Death Company do the same trimming their black armor. Chaplains who have administered the rite of Angel's Ice to a fallen brother are entitled to repaint the skull pattern of their helms from bone to ice-blue, and most do unless their only rites have resulted in the death of the brother's body. Techmarines wear crimson red, but rather than changing the trim, the right shoulder pad and left leg are green. Veteran Sergeants wear ice-blue helmets rather than the traditional but unsightly white stripe on red; decorative patterns in red on top of this are common.

In keeping with the unusual nature of their Death Company, they are not decorated in the traditional way. Rather than black with red trim, they wear the chapter's usual dark green, changing only the trim, which is ice-blue. Chaplains who are shepherds of the Ice-Dead also wear this blue trim, and the Ice-Born officers have helms and torso entirely repainted in blue.

Geneseed

Other than the overactive omophagea common to all of Sanguinius's line, no significant deviations in geneseed are noted. Some magoi believe there is a psychosomatic component to the chapter's omophagea mutation, and the memories gained by the omophagea drain out with the their blood, thus explaining why the ritual of Angel's Ice works for them and not for the few other astartes who have attempted it. Certainly, there are reports from the Ice-Blood astartes that memories gained through the omophagea are crisper than normal, and most report eidetic memory for omophagea-derived memories and those gained during their implantation which does not extend to other memories. The reluctance of the other Sons of Sanguinius to attempt the ritual has severely curtailed experimentation to better determine the cause.

Battlecry

"With the cold vengeance of an Angel!"