I planted the sweet potatoes in grow bags and the big ones look great but I have a bunch of long small potatoes and some of them have this black on the skin. Peeled them and they look fine inside so it seems to only be on the skin. I want to start my own plants this year and want to be sure they will be ok and not spread some kind of disease.
The black skin coloration is scurf. It does infect the soil, so try to avoid soils that have exhibited scurf. http://www.ces.ncsu.edu/depts/pp/notes/oldnotes/no103.htm Some varieties are more prone than others. The potatoes are fine to eat but may transfer spores to slips. I would not recommend using them for seed potaoes. The long small potatoes are normal. Sweet potatoes are roots and not all of them will "beef"up.