As in other species of the family Milacidae, a prominent raised keel runs along the midline of the body from the back of the mantle to the tip of the tail.

[6] Milax nigricans is often abundant around the Mediterranean, especially in disturbed habitats: North Africa, Malta, Sicily, mainland Italy, Croatia, Sardinia, Corsica, southern France, Balearic Islands, northern and eastern mainland Spain, Canary Islands.

In addition, local populations have been introduced further north in Europe and may persist: southern England (one record only), Munich, northern France, the Netherlands, Hungary.

Other records based only on external appearance are liable to be the more widely distributed M. gagates and should be considered unreliable.

There are indications of the long-distance dispersal of M. nigricans on both fresh produce and potted garden plants.