In the wild, the black ocellaris, also known as the Darwin variant, are relatively rare because they are found only in a small area off Australia. The juveniles tend to have more yellow coloration around the snout, mostly, but the yellow is slowly replaced by black as they get older. I've had mine for ~5 months and I've noticed its nose has gotten significantly darker.
Breeding and such has lowered the price and increased the availability of these guys tremendously. FFExpress gets them in quite often for around $30, so you could give them a try.