Described as Cyanomyia sordida (Gould 1859), with a type locality of Oaxaca, Mexico.
Züchner (1999) suggested that Cynanthus sordidus "possibly" formed a superspecies with Cynanthus latirostris (Broad-billed Hummingbird), although Mayr and Short (1970) rejected superspecies status for this pair. Friedmann et al. (1950) mentioned a hybrid between sordidus and latirostris, but Binford (1985) documented that reported hybrids are within the normal range of variation of sordidus.
Phylogenetic analysis of DNA sequence data reveals that hummingbirds (Trochilidae) constitute nine major clades, comprising the hermits, mangos, Patagona, topazes, coquettes, brilliants, mountain-gems, bees, and emeralds (McGuire et al. 2007, 2009). Cynanthus was not included in this phylogenetic survey, but McGuire et al. (2009) suggest that Cyananthus is a member of the emerald clade. Other genera that are documented to belong to the emerald clade are Chlorostilbon, Klais, Orthorhynchus, Campylopterus, Chalybura, Thalurania, Eupherusa, Microchera, Elvira, Aphantochroa, Taphrospilus, Amazilia, Chrysuronia, Hylocharis, Lepidopyga, and Damophila (McGuire et al. 2007, 2009); in addition to Cynanthus, other genera that may belong to the emerald clade include Abeillia, Cyanophaia, Eupetomena, Goethalsia, Goldmania, Leucippus, Leucochloris, Phaeochroa, Stephanoxis, and Trochilus.