Male Black-bellied Gnateaters are distinctive, with black heads and underparts, the head marked with a contrasting white postocular tuft. The back, wings and tail are chestnut. Females have the back and wings similar to the males, but are otherwise different in appearance. Females have a dark gray forecrown with the hindcrown and nape dark brown, and a pale gray supercilium. The throat and center of the belly are whitish with the remainder of the underparts pale gray
The strikingly patterned male Black-bellied Gnateater is nearly unmistakable within its range. Females are somewhat similar to females of the much smaller Hooded Gnateater (Conopophaga roberti) of northeastern Brazil, but have a more prominent postocular brow tuft and more extensive gray on the face and forecrown.
Adult male: In males the entire head, including nape, face, throat, and sides of the neck, is black. The underparts are also uniform black, fading only slightly into dark gray on the lower belly and crissum. The upperparts are a rich rufous chestnut which forms a clean line where it meets the black of the head. The most striking feature is a long, bright white postocular tuft of feathers. Although these tufts reportedly can be flared out laterally, there are no detailed accounts of such behavior.
Adult female: Female Black-bellied Gnateaters have rufous chestnut upperparts, similar to males, but the chestnut extends up onto the nape and crown, becoming duller and browner anteriorly before fading to a gray forecrown. The face, auriculars, and sides of the neck are gray. Below, areas that are black on the male are gray in females, broken only by a white throat and variably sized (and inconspicuous) white belly patch. The postocular tufts of females, though still present, are greatly reduced.
The original description of Black-bellied Gnateater refers to the male, and was published in Latin (Ménétries 1835):
"Castanea; superciliis atque plumulis longioribus ad collum niveis; sutbùs occipiteque nigerrimis; crisso griseo-ferrugineo; alis caudaque nigricante-brunneis, castaneo-marginatis."
Ménétries (1835) also provides a summary in French: "La queue est presque carrée, et ne dépasse les ailes que de cinq lignes. Le dessus de la tête, le dessous du cou, la poitrine et la moitié du ventre sont d'un noir profond; le dessus de l'oeil, ainsi que de longues plumes qui descendent le long des côtés du cou, sont d'un blanc éclatant; le dessus du cou est d'un brun ferrugineux; tout le reste du dessus du corps est d'un marron rougeâtre vif; les ailes et la queue sont brunes, les pennes sont liserées de roussâtre, les côtés de l'abdomen et les couvertures inférieures de la queue sont d'un gris roussâtre."
Iris: dark brown
Tarsi and toes: gray to blue gray
Total length: 13 cm (Sick 1993), 14 cm (Sclater 1890), 14.4 cm (Ruschi 1979), 14.5 cm (Meyer de Schauensee 1970, Ridgely and Tudor 1994, 2009), 14.4-15.8 cm (Whitney 2003).
wings length, 81, 80, 80 mm; tail length, 40, 40, 42 mm; bill length, 18, 18, 18 mm; tarsus length, 33, 33, 32 mm(n = 3, Hellmayr 1910)
wing length, 78 mm; tail length, 42 mm; bill length, 17 mm (n = 1, "immature", Hellmayr 1910)
wing length, 84 mm; tail length, 44 mm; bill length, 20 mm; tarsus length, 33 mm (n = 1, Hellmayr 1907)
wing length, 79 mm; tail length, 39 mm; exposed culmen, 15 mm (n = 1, Novaes 1947)
wing length, 79 mm; tail length, 42 mm; bill length, 17.5 mm; tarsus length, 32 mm (n = 1, Hellmayr 1910)
wing length 77 mm; tail length, 38 mm; exposed culmen, 15 mm (Novaes 1947)
wing length, 73.7 mm; tail, length 38.1 mm (n = ?, Sclater 1890)
wing length, 83 mm; tail length, 38 mm; bill length, 17 mm; tarsus length 36 mm (n = ?, Snethalge 1914)
wing length, 83 mm; tail length, 38 mm; bill length, 17 mm; tarsus length, 36 mm (n = ?, Ruschi 1979; although Snethlage 1914 is not cited, Ruschi likely took these measurements directly from that source)
male: 42 g (n = 1; Graves and Zusi 1990); mean 42.5 g, range 42.0-43.0 (n = 2, Dunning 1993)
sex unspecified: 37-43.5 g (n = ?, Whitney 2003)