[3][4] The blue-tailed emerald was formally described by the Swedish naturalist Carl Linnaeus in 1758 in the tenth edition of his Systema Naturae under the binomial name Trochilus mellisugus.

When he updated his Systema Naturae for the twelfth edition in 1766 Linnaeus added citations including one to the "all-green humming-bird" that had been described and illustrated by the English naturalist George Edwards in his Gleanings of Natural History.

[7][8] The identity still remained uncertain but in 1950 the American ornithologist John T. Zimmer argued that the species Trochilus mellisugus described by Linnaeus could only have been the blue-tailed emerald.

[9] The blue-tailed emerald is now placed in the genus Chlorostilbon that was introduced by the English ornithologist John Gould in 1853.

[3][12][4] Seven subspecies recognised by the IOC are:[3] The Clements taxonomy and HBW include napensis within phoeopygus, considering them indistinguishable from each other.

[13][14][15] However, the IOC, Clements, and HBW do not appear to have fully accepted that treatment, as their linear sequences of species (which reflect relationships) insert the Chiribiquete emerald (C. olivaresi) between the two.

[11] Males of the nominate subspecies have an iridescent golden green forehead and crown, shining bronze-green upperparts, and a steel blue tail.

Males of C. m. subfurcatus have blue-green on the throat and uppertail coverts and golden green on the crown whose intensities are intermediate between the nominate and caribaeus.

[11] The seven subspecies of blue-tailed emerald recognized by the IOC are distributed thus:[3][11] The species occurs in a variety of habitats from the tropical to the temperate zones.

It takes nectar from a very wide variety of low plants, shubs, and trees, and feeds at all levels of the vegetation.

The species makes a cup nest of downy plant material bound with spiderweb and covered with bits of bark and lichen.

The blue-tailed emerald's song has been described as "repeated 'tsip' or chwep notes, with occasional rolls or twitters".