Hard to say with certainty, but it looks like a hen. BSL hens are all black, no barring. If it's a hen it is likely a barred rock, just because they are quite common.
Sex- linked Information - Page 218
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This would be much easier if you replaced your BO's with Barred Rocks, Delawares or Rhode Island Whites. Then the choice of roo is easier - a Rhode Island Red roo could work with both to get black and red sexlinks.
I know, but I coveted those fluffy balls of sunshine for so long, I am entirely unwilling to give them up
It's looking like I have entirely too many cockerels and I'm going to want to cull my scissor beaks, so I was hoping there might be some other option for a roo.
Nope. Sorry, but Buff Orpingtons don't have any of the required genes for feather sexlinks. For that, you would need hens that are 'pure' for the slow feathering gene and a rooster that is 'pure' for fast feathering. It's a very specific set of genes that requires two separate breeding groups to be maintained. And not every breed has the 'fast feathering gene' or the 'slow feathering gene'.