https://replay.pokemonshowdown.com/gen7metronomebattle-972538970
Heal block and Metronome battles
Turn 5: While under Heal Block Buzzwole used Giga drain (was not healed)
Turn 8: While under Heal Block Buzzwole used Heal order (was healed)
https://bulbapedia.bulbagarden.net/wiki/Heal_Block_(move)
Gen IV
Restricted moves can still be called by
moves that call other moves (such as
Metronome), but they are prevented from being used.
Affected Pokémon can still use
HP-draining moves; these moves will still inflict damage, but will not restore
HP.
Gen VI
Affected Pokémon can no longer use
HP-draining moves, except
Leech Seed. Leech Seed will inflict damage to the seeded Pokémon, but will not restore
HP to a Pokémon affected by Heal Block.
I am currently unsure of the mechanics of Heal block + Heal moves summoned by Metronome
According to Bulbapedia it would seem like Heal moves can be chosen and deal damage but be unable to heal (like Giga Drain did) but if a Status heal move is chosen then it would fail
Tests recommended
Test for functionality not using metronome (Yveltal test with Oblivion Wing and Roost to test both attack and status VS a Heal Block pokemon)
Test Timer (it was suggested that it might have something to do with Buzzwole using Heal Order on the last turn of Heal Block)
Test Heal Block vs Metronome spammer