This name is a synonym of Symphyotrichum × amethystinum (Nutt.) G.L.Nesom.
The record derives from Tropicos
(data supplied on 2012-04-18)
which reports it as a synonym (record 2725373) with original publication details: Trans. Amer. Philos. Soc., n.s., 7: 294 1840.
This name record (from Tropicos) 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 Aster amethystinus in version 1 of The Plant List.