I think "dark gray" and "black" can be pretty close calls in this case. Depending on lighting and how close to black you want it to be before you call it black, if you see what I mean. Also, afaik, all Ancistrus species will have a some amount of ability to change colour based on the substrate and/or mood, so if you have dark substrate, the fish will be darker.
If they have spots, they may be
- but it's hard to tell them apart unless the spots disappear...
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Mats