The blue-black grassquit was described by the Swedish naturalist Carl Linnaeus in 1766 in the twelfth edition of his Systema Naturae under the binomial name Tanagra jacarina.

[4] Linnaeus based his description on the "Jacarni" that was described in 1648 by the German naturalist Georg Marcgrave in his Historia Naturalis Brasiliae.

[10] Within the tanager family Thraupidae the blue-black grassquit is in the subfamily Tachyphoninae and is a member of a clade that contains the genera Conothraupis and Creurgops.

[11][12] The blue-black grassquit was formerly placed with the buntings in the subfamily Emberizinae rather than with the tanagers in Thraupinae within an expanded family Emberizidae.

The male is glossy blue-black, with a black tail and wings; the white inner underwing is visible in flight or display.