[3] The sooty barbthroat was formally described by the Swedish naturalist Carl Linnaeus in 1758 in the tenth edition of his Systema Naturae under the binomial name Trochilus niger.

The sooty and pale-tailed barbthroats are recognized as separate species by the South American Classification Committee of the American Ornithological Society (SACC), the International Ornithological Committee (IOC), the Clements taxonomy, and BirdLife International's Handbook of the Birds of the World (HBW).

The adult male has iridescent bronze-green upperparts and breast, a black ear patch and throat, and a pale malar stripe.

As with other hermit hummingbirds, the sexes are similar; the female's bill is somewhat more decurved than the male's and the plumage has less contrast among the throat, breast, and belly.

[11] Little is known about the sooty barbthroat's foraging technique and diet, though it is assumed to be a "trap-line" feeder like other hermit hummingbirds, visiting a circuit of flowering plants.

[11] The sooty barbthroat's vocalizations have not been described, and as of late 2021 no recordings are available at Xeno-canto or Cornell University's Macaulay Library.