where did you get the chick? It looks a lot like my CM/SS mix I hatched earlier this year. The brown flecks in the black feathers are what the BCM Breeders refure to as mossy, color leaking through that shouldn't be there in the pure bred. I'd bet you have a female though... so you have that goin' for you. Other than that though...
If you have more background on the chick that could help a lot. As simple as shipped hatching eggs from a breeder or hatchery chick makes a world of a difference.
shipped eggs from a breeder of mechelse koekoek, chabos, seramas and cochin bantams.
also in the incubator at the time was eggs from my own birds, gold cuckoo marans (only one hatched and it wsnt this chick!)chabos, araucanas and new hampshires
the only black birds the breeder had were black bantam cochins and birchin chabos.
my neighbour (who lives a bout a mile down the road) has a bcm roo.....perhaps he's been visiting?? thats the only possibility i can think of right now..but thats a heck of a distance for the roo to come?
it did not come out of a dark egg.....egg was white or pinky brown if that helps?