I don’t think that is a nigra in the dakotas but a mislabel
www.growingmulberry.org says there are nigra hybrids (that probably could be zone 6)
“It appears that hybridization can occur: A Belgium nurseryman has produced Morus nigra hybrids with Morus kagayame, Morus macroura, and possibly Morus laevigata).”
@Livinginawe do you have more details about the nigra hybrids from Belgium?
Thanks