I think the "accepted" limited range of M. nigra in the US (purportedly between zones 7-9) may have more to do with not enough out-of-zoners having tried than any other reason. Here are some explanations:
1. Many Americans have tasted mulberries that are black (mostly albas or hybrids), but very few Americans have tasted the fruit of the black mulberry Morus nigra, a supreme fruit worthy of obsession. It is a class of its own. No other mulberry comes close. I have tried white albas, black albas and Pakistan mulberries. They are all good, but only distant seconds to the sensory experience of a black M. nigra. So few understand what we're fussing about and why we're going out of our way to widen its range.
2. The reason for 1. is because black mulberries have drawbacks: a. They don't travel well and cannot be stored. When picked, they partially burst and stain everything blood red. So they can only be experienced/sold locally when in season (one or two months a year), which means very few of the at large general public have ever experienced the real taste of M. nigra . b. As the literature warns, they are messy. They are messy to eat. They will stain your hands, mouth and clothes. The stain does not come off easily. Best to wear special clothing for picking, and then eat the fruit with a toothpick. They also stain the area around the tree with fallen fruit that is dragged indoors, and purple bird droppings. That in itself will discourage a lot of casual urban growers. None of that will deter a hard core addict like myself (which means I will keep trying in Miami, zone 10b, and if I fail, I will try again!).
3. The literature/nomenclature is and has been confusing. But thanks to a forum like this one, and Livinginawe's website:
things are fortunately clearing up.
4. Many nurseries are selling N. nigras which are not (see 5). So when these false nigras fruit, and the fruits are nothing to go crazy about, that blemishes M. nigra's reputation, and growers give up on trying to grow the real thing.
5. M. nigras do not propagate easily, either by seed or cuttings. Most if not all trees are grafted. That is a serious limiting factor for its expansion. I have dwarf everbearing (a non nigra) which will root so easily from a cutting, all I have to do is hammer a piece of rebar in the ground, drop a cutting in the hole and walk away...
6. M. nigra is painstakingly slow-growing, another strike against it.
7. Claims that it will only grow in certain climates and stringent hardiness zones. Some of these claims are no doubt substantiated and based on experimentation/trial and error . But others may not be and are just waiting to be proven right or wrong.
8. Microclimates are a reality, not a myth. As in real estate, it's all about location. I have a M. alba (? Florida giant) that has not given me one decent fruit in almost 10 years. A cutting from it planted 5 blocks away at my other property has given me decent fruit in its second year. Just because a nigra fails at one location does not mean it will fail throughout the whole region.
Given all of the above, no surprise it is not as widespread as we would like it to be. Be it as it may, if not for pioneering out of zoners, Florida would not be the mango capital of the US (first introduced in 1833) for instance. My aim is to do the same for Morus nigra. So I applaud k8tpayaso and Livinginawe for trying, and I encourage more people to try. It is well worth the effort...