No idea either I'm afraid but a year or two ago I did get some black mulberry seeds in the RHS membership seed distribution scheme and I now have several babies about 30cm high and ready to be potted on.
I intend growing one as a tree in the main garden so the birds can have fruit, one as a fan (if I can) in a fruit cage we're planning and maybe seeing if I can keep another as a bush. No idea what I'll do with the rest but they won't end up on the compost heap.