I have Heal Block on my bulky Thundurus (until I can find Taunt) and the short description of the move says it blocks the use of items that heal HP, so Lefties would be negatedHeal block does not prevent lefties recovery as well, pretty sure is only recovery moves. The thing is you don't keep a latios on a blissey unless you want to trick her a choice item, but be prepared for the possible status. You could use taunt Hydreigon and U-turn pivot though, as long as you don't switch in blissey obviously.
On topic, after reading the thread i'm conviced that i've been using Hydreigon wrongly, is more suited to open holes on opponent teams than choice-scarf locked revenging stuff, you leave that to latios or garchomp. But specs set works wonder on wifi.
Also on a Hydreigon-related note, you wouldn't even believe how many times the Speed boost from Scarf saved me.