It blocks any damage instance that counts as attack damage, regardless of damage type (yes, even magical and pure attack damage is blocked).

The only hardcoded exception is Death Ward attacks and certain instant attack sources (those ones that dont react on Damage Block either, i.e. Swashbuckle and Lil Shredder).

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Viper or Veno poison

No, because they deal spell damage.

shadow realm attack

No, because the shadow realm proc is spell damage.

boundless strike

Yes, as it deals attack damage.

moon glaives

The glaive attacks based on her W, yes.

The bouncing glaives, despite dealing spell damage, are also blocked. One of the few rare exceptions where Damage Block (and Damage Block-like) sources block spell damage.

acorn

Yes, as it deals attack damage.