This name is a synonym of Pennisetum sieberianum (Schltdl.) Stapf & C.E.Hubb..
The record derives from WCSP
(data supplied on 2012-03-23)
which reports it as a synonym and as an invalid name (record 432831) with original publication details: Consp. Fl. Afric. 5: 781 1894.
This name record (from WCSP) was not included in version 1 of The Plant List. It is possible the same name was included from a different data source, to check this you can search for Pennisetum nigritarum in version 1 of The Plant List.