The International Ornithological Committee and the Clements taxonomy assign it two subspecies, the nominate C. a. antarcticus (Garnot, 1826) and C. a. maculirostris (Dabbene, 1917.

Adults of the nominate subspecies are mostly dark sooty brown, with a slightly paler throat and some dull rufous on the base of the flight feathers.

[8] Subspecies C. a. maculirostris is found in southernmost mainland Chile, both Chilean and Argentine Tierra del Fuego, and other islands in the Cape Horn Archipelago.

[6] The blackish cinclodes feeds mainly on small invertebrates such as amphipods, isopods, flies, and Orthoptera.

Among seabird colonies it feeds on cracked eggs, scraps, and regurgitated matter, and also takes prey from excrement.

It has also been documented feeding at wounds on southern elephant seals (Mirounga leonina) and sometimes enters houses to take food crumbs and scraps.

All of the recordings of blackish cinclodes vocalizations in xeno-canto and the Cornell Lab of Ornithology's Macaulay Library are from the Falkland Islands.

The song is "a long series of sharp, explosive notes intermixed with musical trills...described as tittering chee-chee-chee-chee-cheecheecheecheecheechee-chee".

Some previously affected small islands in the Falklands have been cleared of land predators and populations on them appear to be more stable.