Upon a bit of research, I believe that the Bomgaar's breed list for ordering is consistent with Hoover Hatchery's, so that's where they're likely from. Looking at what they consider rare breeds, it's likely a Black Orpington or a Black Andalusian (only blue are listed, but when breeding two blue breeds together, 25% come out black). You'll be able to tell by earlobe color and body type very easily