If a creature with protection from black is dealt damage by a black source, is the damage prevented entirely?

Protection from a quality grants four distinct shields, often remembered by the acronym DEBT: the protected permanent cannot be Damaged, Enchanted/Equipped/Fortified, Blocked, or Targeted by anything with the matching quality. The damage-prevention aspect is the most relevant here.

Per CR 702.16b, all damage that would be dealt to a creature with protection from black by a black source is prevented. This is a continuous effect — it does not use the stack and cannot be responded to. The moment the game would apply that damage, it is instead wiped away entirely.

This applies regardless of the damage source: a black creature attacking, a black instant like Corrupt, or a black artifact that happens to be black. If the source has the black color identity in terms of its color (not color identity for Commander purposes, but its actual color), the damage is prevented.

Concrete example: Your opponent controls a 4/4 black Demon and attacks. You block with your White Knight (protection from black). When combat damage is dealt, the Demon's 4 damage to White Knight is fully prevented. White Knight takes 0 damage and survives. White Knight's 2 damage still resolves normally, dealing 2 damage to the Demon.

Note that protection does NOT prevent the creature from being affected by non-damage effects from black sources — for example, a black sorcery that says 'destroy target creature' cannot target the protected creature in the first place (due to the T in DEBT), but that is a separate shield from the damage prevention (CR 702.16b, 702.16e).