Hard to say breed or variety for sure.. but definitely not cuckoo as it would have a white spot on the head at hatch and be showing barring by now.
*Possibly* a midnight majesty maran.. which is a black sex link crossed between black copper marans rooster and barred rock hen.. the females would be all black with no spot on the head or barring. Seriously though I'm just making a wild guess here. Are the legs clean or feathered?
The middle man sellers can only tell you what the hatchery has told them. Most will not be able to truly identify the breed or variety. You may be able to find out which hatchery they order from, which can sometimes help narrow down breed possibilities.. IF it is MMM.. the pullet offspring may get the egg color of their father as sire's egg genetics effect that more so than the dames, according to my current understanding.