The blackout dates work as expected.
I saw WestJet post their 26% sale (promo: FLY1996 if you wanna check it out) last night and they mis-coded Easter Sunday (Apr 17) as a blackout date when their T&C specified that they meant Monday, Apr 18 to be the black-out. I need to travel on Apr 17 so I'm glad I woke up to an almost-matched AC sale for this date.