In birds, sexual selection on plumage may promote sexual dichromatism and male ornamentation. However, even in drab species without obvious sexual dichromatism, plumage may still be used for mate choice. A previous study found weak but significant sexual size dimorphism in North American Black Terns (Chlidonias niger surinamensis), but no assortative mating based on morphology. However, the relevance of plumage variation to mate choice in this species is yet untested. Here, using reflectance spectrometry and avian visual modeling, we revealed that Black Tern males and females exhibited a subtle but significant difference in brightness (males darker) in a midwestern USA population. Importantly, the achromatic contrast between the sexes was perceptible during mate choice. Moreover, there was evidence of disassortative mating for plumage color as measured by hue and saturation, but not brightness. There was no relationship between plumage color or brightness and body mass. This finding suggests that visual signals unrelated to body condition may be relevant to Black Tern mate choice. The pattern of disassortative mating was unexpected, and potential benefits of choosing a mate dissimilar from oneself are discussed. This study highlights the importance of considering the avian visual system when studying plumage variation elusive to human observers.