There are about 6 weaver species that have completely black plumages. In one case, only the female is black (Cassin's Malimbe, not illustrated here). The 2 buffalo weavers are large and are savanna species, while the others are generally forest birds. Several weavers are largely black, but have patches of colour that help identify them, eg Golden-naped Weaver and the widows and bishops.

White-billed Buffalo-Weaver Bubalornis albirostris - white bill
Red-billed Buffalo-Weaver Bubalornis niger - red bill
Vieillot's Black Weaver Ploceus nigerrimus - yellow eye
Maxwell's Black Weaver Ploceus albinucha - whitish eye
Yellow-legged Weaver Ploceus flavipes - yellow legs
Cassin's Malimbe Malimbus cassini - female - dark eye