Posting @Unity’s question here
So the first thought I have: it may seem unthematic, but there’s generally small edge cases that are correct by the rules, even if they don’t quite match the themeing. The important details are that Blackhand has a requirement (the hero has to have died previously during the game) and an effect (the hero is brought into play at max level).
Second thought: this seems like a case of “specific trumps general”; Snapback blocks all summons, Blackhand allows a specific type of summon, so that overrides Snapback. I could be wrong about that, though.
EDIT: don’t mind me, the better rule to follow is probably “‘can’t’ trumps ‘can’”, so Snapback would block Blackhand’s effect.