What do you mean of the sort hard to tell? Could blacks be yellow with grey backs like the blues would be??
It could be that hen or Roo is a Recessive black!? I have a recessive white hen Who is actually a barred cochin, I found that out when i bred her to my EE Roo and hatched out sexlink chicks barred boys and non barred hens. As it was explained to me think of it as you chicken is actually blue but is wearing a black over coat that covers it up. Or maybe it is a really dark navy blue that looks black, I Bought some blue cochin chicks that looked just like a black chick should, but when they started feathering out it was a really dark navy blue. Looks black in some light and blue in others!
I am not a genetics expert but i got a lot of help when i posted in the genetics section, you should do that with a pic of the parents and the chicks hatched from them.