I hate it. Not asking for advice, I’ve argued about this enough. But I feel blue doesn’t need the ability haste, because he should delete a target in one rotation, and shouldn’t be “poking” or teamfighting. And his damage doesn’t spike as much as I’d prefer with youmuus, I prefer straight to duskblade, manamune, youmuus 3rd. Mobi boots to make up the engage and chase speed lost by not rushing it early. The fact you don’t need Ionians, opens you to mobis again, and that opens you to not needing the movespeed off of youmuus as much, and can safely go the higher damage duskblade, plus the extra invis to help escape after a pick. Duskblade passive gives ability haste, which, while I’m adamant isn’t a great stat on blue, is worth it in small doses, due to duskblade having as a passive, rather then building specifically for it. Duskblade should be more then enough ability haste by itself in his core, thus not needing Ionians.

TL;DR: going mobis instead of Ionians lets us go duskblade first and get youmuus later. Duskblade lets us not need Ionians at all(although I am of the opinion that they weren’t needed in the first place), and frees us up to go mobis. A synergy loop.