This name is a synonym of Juncus balticus subsp. ater (Rydb.) Snogerup.
The record derives from IOPI
(data supplied on 2012-03-23)
which reports it as a synonym (record 316085) with original publication details: Fl. Rocky Mts. 152 1917.
Full publication details for this name can be found in IPNI: http://ipni.org/urn:lsid:ipni.org:names:132366-2.
This name was in version 1 of The Plant List, record kew-316085.