Yes, those should be Morus nigra. All nigras have compact growth, but are typically grafted up high so they most have a tree form and presumably could reach ten or fifteen feet. The dwarf variety should be a genetic dwarf though. I would not consider a nigra if you want a ton of berries though, the ripening is very spread out. They are also way messier to pick, and can be too sour for some people, though I think they are amazing when dead ripe!