Before Gen VI
Heal Block merely stops the target from healing, but does not prevent leeching moves from doing damage or being used. As you can see [here], Giga Drain still damages Tentacruel. Also, Venusaur still takes the damage from Liquid Ooze.
Heal Block now prevents the moves Absorb, Draining Kiss, Drain Punch, Dream Eater, Giga Drain, Horn Leech, Leech Life, Mega Drain, Oblivion Wing, and Parabolic Charge from being used. Leech Seed can still be used and will still inflict damage, but will not restore HP.
In summary, before Gen VI Heal Block and Liquid Ooze will still damage both the Pokémon using the leeching move and the target of the leeching move. However in X and Y, Heal Block prevents the blocked Pokémon from using and leeching moves.