Fun Fact: Heal Block stops you from healing b everything accept for Z-moves. and the one affected by Heal Block can not use Draining Moves like Draining Kiss and such
Bulbapedia said that, in gen 7, only Z-moves like Z-softboiled can heal the mon affected by Heal Block. Leech Seed can, actually, be used. BUT You won't regain the HP you drain, only deal the damage