Both sexes have very similar appearances with black plumage and a large white patch on the upper side of their wings in summer.

Eggs are a dull white to pale green in color, boldly marked with dark spots and blotches.

The black guillemot was formally described in 1758 by the Swedish naturalist Carl Linnaeus in the tenth edition of his Systema Naturae.

[7] Five subspecies are recognised:[5] The black guillemot is a medium-sized bird with adults normally 30 to 32 centimetres (12 to 12+1⁄2 inches) in length and with wingspans of 52 to 58 cm (20+1⁄2 to 23 in).

Juveniles and immatures can easily be identified by the spotting of the white wing patch with grey or brown feathers and is easy to see even at far distances in the field.

Typically restricted to rocky shores, black guillemots utilize the cliffs, crevices and boulders for their nests, hunting the inshore waters for benthic prey.

Compared to other auks, they forage fairly close to the colony, in the breeding season mostly in inshore waters more than 50m in depth, farther afield in the winter months.

One of the early ornithologists that described aspects of the behaviour of the black guillemot was Edmund Selous (1857–1934) in his book The Bird Watcher in the Shetlands (1905).

[8] In the chapter titled 'From the Edge of a Precipice'[9] he writes for instance that sometimes the black guillemots carry a fish they have caught in their beak for hours.

[2] Guillemots are single-prey loaders, meaning they bring single prey items back to their chicks during the chick-rearing period.

This limits the spatial range that parents can forage for food, as chicks must receive a high number of energy-rich prey items throughout each day.

[10] Black guillemot diets include sculpins, butterfish, rock gunnel, northern sandlance, herring, jellyfish, mollusks, and other small crustaceans.