This tiny (4mm) metallic blue fly, Condylostylus mundus, is a member of the 1,275 species family Dolichopodidae and is a valuable predator in any garden. They eat great quantities of mites and aphids. This one, blue, is a male....the females are metallic green.
I believe this to be a female....but the genus Condylostylus has 40 species in North America, so I could be mistaken.
The wings are hyaline and without pattern and the legs are all black. They have a domestic range from Florida north to North Carolina and are also found in the West Indies and Brazil.