I'd email the breeder and ask what breeds are in their sex links. Sex links can be a cross of pretty much any breed, as long as the hen is barred. For example, a black rooster over a legbar hen would give black sexlink pullets that have a crest. Cuckoo silkies or barred polish could also give a crest. Or maybe she's not a F1 cross, a barred olive egger bred to a solid rooster could also give you black sexlinks.