Sunbeams (Aglaeactis) are large hummingbirds with a straight bill of medium length (for a hummmingbird), and which occupy high elevation humid forest edge in the Andes. In sunbeams the throat is dull, with no glittering gorget, but all species of sunbeam have a tuft of white or rufous feathers on the breast, and the rump usually is some shade of iridescent purple. The sexes are similar, but the iridescent rump is much reduced in females. In Black-hooded Sunbeam, the body is mostly purplish back, with a glittering blue or green lower back and rump, and white breast tufts. The tail is rufous.
Black-hooded Sunbeam is very distinctive within its restricted geographic range, and is unlikely to be confused with any other species. It is the only species of sunbeam that occurs in Bolivia. No other hummingbird within its range has the white breast tufts, glittering blue green rump, or rufous tail. Black Metaltail (Metallura phoebe) is superficially similar, but does not overlap geographically with Black-hooded Sunbeam; also, the metaltail is almost entirely black (with no glittering rump or breast tufts), has a small glittering gorget, and the rectrices of the metaltail are redder, less rufous.
The following description is based on Fjeldså and Krabbe (1990) and on Fjeldså (1999):
Adult male: Plumage primarily purplish black. Lower back and rump glittering golden green to bluish green. Rectrices rufous, with narrow dusky tips. Tuft of white feathers on the center of the breast. Tail slightly forked.
Adult female: Similar to adult male, but duller, with less iridescence on the rump and back, and the tail is less forked.
Juvenile: Slightly browner than adult; breast tuft often inconspicuous. Rectrices more olivaceous, but "apparently shows metallic colors on rump from an early age" (Fjeldså and Krabbe (1990).
Bill: black in adult; mandible orange in juvenile (Fjeldså and Krabbe 1990)
Tarsi and toes: colors undescribed?; probably dark brown or blackish
Total length: 12 cm (Fjeldså and Krabbe 1990)
Bill length: 15 mm (Fjeldså and Krabbe 1990), 16.9-19.2 mm (Colwell 2000)
Mass: male, 5.2 g (n = 1; Colwell 2000); female, 5.8 g (n = 1; Colwell 2000)