Black Rocks are from the Muirfield hatchery near Kinross and they'll only sell pullets or hens, not cockerels. So you won't get a Black Rock Cockerel unless you're a breeder who is registered and licensed by Muirfield to breed and sell Black Rocks on their behalf. It's from two particular strains of RIR and Plymouth Rock that have been bred specifically for the purpose for several generations.
In theory if you had another RIR x PR cockerel and hens that you had bred yourself then you could get chicks but they would just be hybrid chicks, they could turn out like anything really, like Sandy says. Bet they would be nice looking and good layers though.