Adult males of Black-bellied Seedeater usually have a bluish gray dorsum, cheeks and flanks in the breeding season. The throat, chest, belly, and undertail coverts are black. This is the only black and gray seedeater in the "capuchino" group of Sporophila.
The most common plumage pattern of the Black-bellied Seedeater is similar to the Chestnut-bellied Seedeater of Amazonia (S. castaneiventris) with the color of the throat and center of underparts being the primary difference. Male Black-bellied Seedeaters with a rare plumage morph, the xumanxu form, have a plumage pattern that is similar to that of males of the Chestnut Seedeater (S. cinnamomea), in which the color of the throat and breast also extends onto the sides of the head and neck, as well as onto the back; but color remains black, as in standard Black-bellied Seedeaters, rather than the cinnamon/chestnut of Chestnut Seedeater (Repenning et al. 2010).
Male. Males have a mostly bluish gray plumage, throat and center of breast and belly black (median underparts). Wings and tail are blackish gray, wings with a white speculum (white patch at the base of the primaries). Males show a great variability in plumage, some with black cheeks and flanks, reaching an extreme with the whole plumage black and a bluish-gray forehead and crown (xumanxu form), although the latter is less frequent (Repenning et al 2010). Young males (before one-year old) and non-reproductive males show a plumage pattern olive brown. Juveniles (male) younger than a year old are female coloration
Female. The female is much less distinctive, predominantly olive brown above and paler buffer brown below. Wings with whitish speculum as males, similar to many other species of capuchinos.
Preliminary studies show that Black-bellied Seedeater undergoes a complete postbreeding molt (prebasic). It starts before the autumnal migration or during the first migratory displacements to north. In the complete molt, males change from the alternate (nuptial) plumage (black and gray) to a drab ("eclipse") nonbreeding plumage. A discrete molt of contour feathers can be observed still on wintering sites or stopovers to breeding sites. In such molt, black and gray feathers gradually replace the drab plumage. The females undergo the same process, but a new drab plumage replaces the older one (Repenning and Fontana unpublished data). Preliminary studies show that definitively plumaged males (i.e., males that are 2+ years old) have eclipse processes at random. Sometimes males do not reach the complete nuptial plumage and in the next breeding season reacquire the complete breeding plumage (Repenning and Fontana unpublished data).
The bill of breeding males is usually black or dark gray. During the non-breedingplumage, i.e. when birds are displaying the drab plumage, the bill turns from drab pale to yellow overall at the base of the mandible. The bills of females and immatures are usually brownish gray, with the base of the mandible is yellowish. Tarsus and nails are dark and iris is brown.
Body mass: 8 – 10.5 g. Total length: 100 – 106 mm (Belton 1994, Rovedder 2011). Females tend to have lower wings and tail slightly smaller than males, although significant differences have not been reported. There are no morphometric differences (i.e. in body size, body mass, beak) between males and females, although the tarsus is significantly shorter in females (Rovedder 2011).