If I have bloodhost on the battlefield and he and all my creatures have protection from black (like from akroma's memorial), can he still use his card ability to sacrifice another creature and gain a +1/+1 counter? Or does protection from black, protect the other creatures and stop his card ability?
Blood Host's ability does not damage, attach to, block, or target the creature that's sacrificed. You will be able to sacrifice a creature with protection from black to activate the ability.
Private Mod Note
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DCI Level 2 Judge
Scientists have calculated that the chance of anything so patently absurd actually existing are millions to one. But magicians have calculated that million-to-one chances crop up nine times out of ten.
Just as 'Indestructible' is not 'absolute invulnerability', the word 'protection' has a specific game definition, somewhat different from its regular English definition. Let's not make assumptions which are not backed by the rulebook!
Protection is defined by rule 702.16:
702.16b A permanent (…) with protection can’t be targeted (…)
702.16c A permanent (…) with protection can’t be enchanted (…)
702.16d A permanent with protection can’t be equipped (…)
702.16e Any damage (…) to a permanent (…) with protection is prevented.
702.16f Attacking creatures with protection can’t be blocked (…)