You might be onto a loser with a black rock. My MIL has them and has one that has brooded (and hatched) twice - at her own initiation - but none of the rest have shown any signs. My Orpingtons were all broody at some point last season - the first went broody in February and sat tight while snow was falling around her (I brought her into the house when she was literally covered in snow). The rest were all, sensibly, later. So I would wait until April/May time and then leave a clutch of fake eggs in one of the nest boxes and see if you get any takers. Mine would be piled on top of each other but you'll just have to see with yours.
H