The male's face and most of its underparts are metallic green; the undertail coverts are blue-black or black with a bluish gloss.

[1] The black-billed streamertail forages for nectar at a wide variety of native and introduced flowering species; it especially prefers Besleria lutea.

It has also been observed "robbing" nectar from holes in flowers created by bananaquits (Coereba flaveola) and visiting wells drilled by yellow-bellied sapsuckers (Sphyrapicus varius).

In addition to nectar, it feeds on small insects taken while hovering or gleaned from foliage or spiderweb.

Both sexes make repetitive forward and back flights, after which the male flies up and down in front of the female while spreading its tail streamers.

The black-billed streamertail's vocalizations include "a loud metallic ting or teet and a prolonged twink-twink-twink dropping in pitch at the end."

The whirring is synchronized with the wingbeats and video footage shows primary feather eight (P8) bending with each downstroke, creating a gap that produces the fluttering sound.

[8][9] The IUCN has assessed the black-billed streamertail as being of Least Concern, though it has a restricted range and its population size and trend are not known.