The blackish-blue seedeater (Amaurospiza moesta) is a species of bird in the family Cardinalidae, the cardinals or cardinal grosbeaks. It is found in Argentina, Brazil, and Paraguay.

The blackish-blue seedeater is 12 to 12.5 cm (4.7 to 4.9 in) long and weighs 12.5 to 14 g (0.44 to 0.49 oz). The male is entirely dark slaty blue that is darker on the face and underparts. The female is tawny brown, with the underparts paler than the upperparts.