Icy Wind is illegal most likely because it's literally impossible to get in-game (At least, along with certain moves or abilities).
It's not a matter of banning and in general Smogon makes sure not to ban moves, and rarely bans abilities or does combination bans to avoid such confusion.
Seeing as Blissey has Ice Beam available and doesn't outspeed much even after a speed drop, you might want to just use Ice Beam instead.
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