My best guess is Keeper of the Dead, a black uncommon from before 8th edition with an activated ability, and its art is decently similar to Bog Initiate. Saying this has "an ability set similar to Cromat" is a stretch, but it's by far the best match I could find from an art perspective and its repeatable removal could make it highly sought-after especially if inexperienced players neglect to enforce the extra conditions well.
Knight of Stromgald is an uncommon mono-black creature from before 8th edition, with multiple activated abilities, and its art shows a single human standing upright centered in front of a monochrome background.
Nightscape Master is a rare mono-black creature from before 8th edition, with multiple activated abilities using different colors of mana and its art depicts a single humanoid (technically a zombie, not a human). This a stretch to say the art is "similar to Bog Initiate", but its ability set is more similar to Cromat than the others.
Lastly, Avatar of Woe is a rare mono-black creature originally printed before 8th edition, which has the same artist as Bog Initiate in its initial printing if "similar to Bog Initiate" was more a matter of art style than composition. It has several abilities, though they aren't very similar to Cromat's, and I could see it feeling overpowered if playing casually with friends at school.
If none of these is correct, here is all black creatures with activated abilities, with alternate arts shown separately sorted oldest to newest. The card you are looking should be somewhere in there unless your "ability set similar to Cromat" description was wildly inaccurate.