The male has glossy blue-black upperparts, with a scarlet throat and breast, a dark grey upper belly, olive flanks, a white lower belly, and a buffy vent and undertail coverts.

It builds a nest of moss, camouflaged on the outside with lichens and lined with the pith of tree ferns.

Though its numbers have not been quantified, the black-sided flowerpecker is said to be common throughout much of its range, and any declines are not thought to be precipitous.

When Richard Bowdler Sharpe first described the black-sided flowerpecker in 1887, using a specimen collected on Borneo's Mount Kinabalu, he named it Dicaeum monticolum.

[3] The genus name Dicaeum is an Ancient Greek word, perhaps for an Indian bird mentioned by Aelianus.

[4] The black-sided flowerpecker is a very small passerine, measuring a mere 8 cm (3.1 in) in length.

His face and the sides of his neck are black, his chin is white, and he has a scarlet throat and breast surrounded by a greyish-black border.

[8] The female has olive-green upperparts and greyish underparts, with buffy flanks, a yellowish rump, a whitish throat and white pectoral tufts.

Young have been recorded between November and February, and adults in breeding condition have been found during that period.

[8] The International Union for Conservation of Nature rates the black-sided flowerpecker as a species of least concern, though its population has not been quantified.

[4] Destruction of forest for oil palm plantations could have a detrimental impact on their numbers.