OK, I did some digging and I think I have two possibilities for you. It could be a black leghorn, or it could be a black sex link. I think that (depending on where you got them from) the black sex link is more likely, it fits with your description better (its hard to actually see the chick, the picture isn't very good). If your other chicks have yellow on the tops of their heads then they probably aren't Back Jersey Giants? Black Jersey Giants tend to have a lot of yellow in their coats as babies. If they have a little yellow stripe or spot right on top of their head with almost no yellow anywhere else then they are probably Barred Plymouth Rocks. Hope that helps!