Good question! I found this:
Gasteria pseudonigricans Haworth 1827
"This doesn't appear to be a vaild name despite this succulent plant appearing in many collections. Could be G. subnigricans which may in turn be a form of G. brachyphylla or just an attractive plant of garden origin."
(page down near the bottom)
The Plant List:
"Gasteria pseudonigricans Haw. is an accepted name. This name is the accepted name of a species in the genus Gasteria (family Xanthorrhoeaceae).
The record derives from Tropicos (data supplied on 2012-04-18) which reports it as an accepted name (record 18402513).
Synonyms: No synonyms are recorded for this name."
Gasteria pseudonigricans (Salm-Dyck) Haw.
National Status: Status and Criteria: Not Evaluated
Justification: Taxonomic problem/doubtful taxon.
SANBI. 2015. Gasteria pseudonigricans (Salm-Dyck) Haw.
National Assessment: Red List of South African Plants version 2015.1.
Accessed on 2015/12/04