Considered Near Threatened by BirdLife International, Black-legged Dacnis is endemic to southeast Brazil, where it is known from Espírito Santo south to Santa Catarina, and from sea level to 1700 m. The male is mostly turquoise blue, with a black throat patch, short eye stripe, and upper mantle, and mostly black wings with broad blue fringes to the wing coverts and secondaries. It is very similar, therefore, to the same sex of Blue Dacnis (Dacnis cayana), which has more extensive black on the back and throat, narrow blue fringes to the primaries, and pinkish or red (rather than black) tarsi. In contrast, female Black-legged Dacnis is a much more readily identifiable, being brownisholive above, tinged greenish blue over the forecrown, cheeks, scapulars and rump, and dull buffish below, whereas female Blue Dacnis is bright green with a bluish head. Black-legged Dacnis appears to move seasonally, or perhaps erratically, probably in search of favorite food-plants. The diet is known to include berries, seeds, insects, and even eucalyptus nectar. Until recently, little was known concerning its breeding and feeding behavior, in part due to its rarity, but perhaps also due to a lack of knowledge of how to identify it in the field.
The genus name Dacnis comes from the Greek word daknis, which refers to an unspecified Egyptian bird, and the specific epithet nigripes comes from the Latin words niger, which means black, and pes, meaning foot (Jobling 2010). In Portuguese the common name is Saí-de-Pernas-Pretas (CBRO 2010), and in Spanish the common name is Dacnis Patinegro (Hilty 2011, de Juana et al. 2012). Thus, the scientific name and common names of this species all describe the distinct black tarsi, which are an identifying characteristic for the species.