Blackburnia kavanaughi, new species, previously misidentified as Blackburnia asquithi Liebherr, is described from the western portion of the Alakai Plateau, Kauai, Hawaii. The species is placed phylogenetically based on cladistic analysis including all known Hawaiian Blackburnia, supporting membership in the subgenus Metromenus Sharp. The new species is hypothesized to be most closely related to B. debilis (Perkins) of Molokai and B. kuiki Liebherr of East Maui, sharing enhanced pubescence of the antennal pedicel, reduction in number of the dorsal elytral setae, and elongate ventrolateral setae of tarsomere 5. Individuals of all three species reside in deep arboreal moss mats associated with ohia lehua trees (Metrosideros polymorpha: Myrtaceae). Blackburnia debilis, previously known only from the single male holotype collected in 1902, was recollected in 2005 from Uapa summit, Molokai, in arboreal moss on the mesic leeward edge of ohia lehua forest. Discovery of B. kavanaughi on Kauai implies an additional colonization event from Kauai to newer islands during the diversification of Blackburnia, while corroborating the previous general biogeographic pattern of progressive colonization of the Hawaiian Islands by this clade. Phylogenetic placement of B. debilis B. kuiki as adelphotaxon to B. kavanaughi results in a hypothesized sister-group relationship between sympatric sister species—the epigean B. calathiformis (Sharp) and the troglobitic B. howarthi (Samuelson and Liebherr)—corroborating F. G. Howarth's general hypothesis for the evolution of Hawaiian troglobites.