Sex-Links are hybrids bred to be color sexed at hatch. One common cross is RIR x RIW, which makes red pullets and white cockerels. Another common cross is RIR x BR, which makes black pullets and barred cockerels.
FYI, this is an F1 cross. If you breed two Sex-Link birds together, you'll get mixed chicks that are not color sexable.
The chick color pattern may or may not resemble the adult color pattern. The sex link is handy because the baby chicks can be sexed by appearance immediately. The black sex link is from a RIR rooster and a BR hen. The male chicks have a white spot on the head and the females are all black. Another neat byproduct of the cross is increased egg production...the BSL's are egg laying machines!!