It varies depending on the breeder, hatcheries usually keep their parent breeds a secret. Generally it's a barred female and a red male.
According to feathersite.com, Black Sex-Links are the result of a Barred Rock female and a RIR or NHR male. I don't know if this is always the case, though, and as Whirlwind pointed out, you don't need to use those specific breeds to result in a Sex-Link. Sex-Links are hybrids that are bred to be able to be sexed easily as chicks. But not every cross will result in a good layer, which is what most hatcheries are also trying to get when they breed hybrids a good-laying chicken that can be sexed at hatch.
Look over on the Breeds, genetics, and showing sec. at the top under the sticky thread Sex-links by tadkerson. There's a lot of great info there, I think you should get your aswer there. catdaddy
I read the title and thought BSL... their talking about Breed Specific Legislation on BYC? BTW, that BSL is when there are laws designed with certain breeds in mind...
