This is one of those cases where there are two hostas with the same name. One is registered and, therefore, the "official" version of H. 'Black Forest' while the other is a non-registered plant and needs to be renamed to avoid further confusion.

This cultivar of unidentified parentage was originated by Chet Tompkins of Oregon and registered on his behalf by Peter Ruh of Ohio in 2005. The plant is medium to large in size with dark green foliage. Deep lavender, bell-shaped flowers bloom on tall scapes.

Hosta Helper will call this plant 'Black Forest' (Tompkins)

In The Genus HOSTA by W. George Schmid (1991), this cultivar, or at least one with the same name is listed as having been originated by Royall Bemis of Blackthorne Gardens in Massachusetts. Unfortunately, there is no description included.

Hosta Helper will call this plant 'Black Forest' (Bemis)