No other swift in the U.S.A. approaches the Black Swift in size. Chimney and Vaux's Swift are similarly patterned but are paler about the throat and breast, and have very short, blunt tails. White-throated Swift is more similar but is easily distinguished by its white throat and flank patches.
Length and wingspan from: Robbins, C.S., Bruun, B., Zim, H.S., (1966). Birds of North America. New York: Western Publishing Company, Inc.