The blue seedeater was formally described in 1861 by the German ornithologist Jean Cabanis based on a specimen that had been collected in Costa Rica.

[9] Two subspecies are recognised:[6] The blue seedeater is 11.5 to 14 cm (4.5 to 5.5 in) long and weighs 12 to 15 g (0.42 to 0.53 oz).

The male A. c. relicta is more slaty (less blue) than the nominate and has black lores; the female is a paler cinnamon.

A. c. relicta is found in six southwestern Mexico states, Jalisco, Colima, Morelos, Puebla, Guerrero and Oaxaca.

It contained two seedeater eggs and one of the brood parasite bronzed cowbird (Molothrus aeneus).