Could be a black sex link. A buff orp roo over a barred rock (or any barred female) will produce black sex links. Males will have a white dot on their head and feather in barred. Females will not have a dot and feather in black with some brown leakage.
Edited: Just saw that the breeder only has Buff orps. If that's the case, then I don't know.
Have you contacted the seller of the eggs? If so, do you mind sharing what they said - I am very curious.
I do wonder about the hatchery angle. I have a hatchery Buff Orp and when crossed with my Splash Orp - they produce blue chicks with buff leakage/lacing - but never a black or solid blue (chicks have blue body and buff face at hatch)
eta: spelling errors I caught - brain and fingers not connected this am.
I'll grab some pics as soon as they come out of the bator in a bit...still have a couple hatching. I can't catch the chick in question through the viewing windows of the bator
The person that the eggs came from only raises and shows the Buffs...I sent her an email to ask her about it a little while ago, but probably won't hear back from her until this evening. Her flock is SQ and originated from her grandfather's stock..so no hatchery ties.
i can say after breeding buffs for 7+ years including orps, silkies and ameraucanas, never once anything other than buff hatchlings here. they must have other birds id think or a nabors chicken possibly crossed with the sellers....