My guess, a cockerel. That is very odd that you would have a black chick with a barred pattern like that come out of a Leghorn and Golden Comet.
The spot is a dead give away for a Barred Rock, but with the parentage you are stating, this is odd. But there are couple of other things that are making me lean toward cockerel. The legs look like they are pretty hefty for a pullet, especially the breeds you are stating. The comb, though both the parent breeds have larger combs on the girls as well, this seems pretty large for girls of even those breeds. The other thing is the way it is carrying itself. What is the chicks behavior? When you hold your hand down over top of the chicks, does it coward a little or does it reach up to see what is going on? Sometimes that is a good indicator of males, the ones who reach up.
The only thing that would make me hesitate is that it is feathered pretty well for that age, which often indicates pullet.
Good luck either way.
And, are you sure there is no way a Barred bird could have sneaked in this mix? You know it takes about a month for a hen to be "clean" from previous suitors when you put them in with a new roo?