Barely any Pokemon learn it. The best are dialga, kelfki, giratina, Palkia, latios, celebi,thunderous, silvally, type null, and bronzong. 5 of which are banned, one with prankster, meaning a dark type makes it fail, leaving silvaly, type null, celebi, and bronzong. Silvally and type null aren't very good in general, with type null not being fully evolved and slow and silvally having to be mixed because of an interesting movepool. If it isn't mixed, normally its multi attack, parting shot, defog, and something. Heal block is a waste in that last slot, a coverage move is better. Bronzong is an interesting one, but it isn't amazing unless it's a trick room team, otherwise there's no reason to use it purely for heal block. Celebi is the one who isn't that bad, but it has a ×4 weakness to bug, taking large damage from you turn. It's the one Pokemon who wouldn't be bad for it, but 1 Pokemon with it just doesn't make it popular. The other reason is the fact that it only lasts five turns. They can just keep switching, and when it's over they can just heal back up. So, while it's a not bad move, a good amount of factors contribute to why it isn't very good. Also, there is this source that shows all the Pokemon with heal block of you wanna see it. Hope this helps!