This small swallow is appropriately named for its appearance. Its upper parts are a glossy steel-blue to violet-blue. Its wings and tail are blackish with steel-blue margins. Below, it is white with a gray wash on the sides and flanks. The Blue-and-white Swallow's tail is short and slightly forked. Its wings are shorter and broader than those of migrant swallows. The sexes do not differ in appearance.
The Blue-and-white Swallow is similar to Mangrove Swallow (Tachycineta albilinea), Tree Swallow (Tachycineta bicolor), and Violet-green Swallow (Tachycineta thalassina). Its black undertail coverts distinguish it from these swallows. It also resembles the Pale-footed Swallow (Orochelidon flavipes), which has a pinkish brown throat and darker sides (Turner and Rose 1989).
The Blue-and-White Swallow adult plumage is characterized by glossy steel-blue on the forehead, crown, mantle, back, rump, and uppertail coverts. The mantle feathers are spotted under the surface with white or pale gray. The feathers over the ears and lores are a less bright blue. The wing coverts are black with steel-blue margins that are not as distinct in the greater coverts (Turner and Rose 1989). The underside of the swallow is white with a gray wash on the sides and flanks. The white underneath can be warm buff in fresh plumage. The far sides of the breast are black. A few dusky spots on the chest of some birds can also be pronounced and form a narrow collar (Turner and Rose 1989). The undertail coverts are black with steel-blue margins, and the underwing coverts and axilaries also are gray with whitish tips (Turner and Rose 1989). The tail feathers are grayish black, and the tail is slightly forked. The only difference in adult plumage between males and females is that the female tends to have duller white and stronger buffy coloration (Stiles and Skutch 1989).
The juvenile's upperparts are gray-brown, often with some bluish gloss (Stiles and Skutch 1989). It has a dull sooty-brown crown and back. The tail and wings are blackish-brown, while the china and throat appear white with a buff hue. The breast band and sides are gray with a buff hue. The undertail coverts are dull brown, sometimes with white tips. The rump feathers show narrow, pale tips. The tail tends to be less forked than the adult (Turner and Rose 1989).
Molt of Blue-and-white Swallows begins near the end of the breeding season (Turner and Rose 1989).
The Blue-and-white Swallow's bill is black, and its feet are blackish. The eyes are brown.
The following measurements are from Turner and Rose (1989).
Total length: 12 cm.
Wings: 88-102 mm (mean 94.2 mm).
Tail: 42-55 mm (mean 49.7 mm); fork in tail 7-15 mm (mean 10.6 mm).
Bill: 6-8 mm (mean 7 mm).
Tarsus: 9.1-11 mm (mean 10 mm).
Mass: 9-13 g (mean 10.5 g).