90% or more of US nurseries &/or big box stores selling 'Black Mulberry' are not selling true M. nigra, but rather, in most cases, some black-fruited M. alba selection. Unless it's coming from a CA or PNW source, I can almost guarantee that they are NOT M. nigra/Black mulberry.
M.nigra wants a Mediterranean climate, is difficult to root, and is very slow-growing on its own root...somewhat more vigorous grafted onto seedling M. alba rootstock.
Pear pictured is one I found in Tallapoosa Co. AL, nearly 15 yrs ago. I noticed the ortet on Jan 1, loaded with fruit. This pic of my oldest grafted tree of it, taken on 23 Nov., fruits shaken out on 3 Dec.