The Black-tailed Gnatcatcher is so small but long-tailed that it is only likely to be confused with other gnatcatchers. Blue-gray Gnatcatcher has a whiter tail when viewed from below and lacks the brownish wash to the wings. Blue-gray Gnatcatchers lack the black cap of the male Black-tailed Gnatcatcher. California Gnatcatchers are grayer below, have less white in the tail and have a different call. The very local (Southeast Arizona) Black-capped Gnatcatcher has more white in the tail when viewed from below, a longer bill and males have a downward black projection at the rear of the cap.
Length and wingspan from: Robbins, C.S., Bruun, B., Zim, H.S., (1966). Birds of North America. New York: Western Publishing Company, Inc.