The Black Arms are an alien race, seen only in the video game called, Shadow the Hedgehog.
The Black Arms have crimson and jet black skins. As quoted by Doom's Eye in Shadow the Hedgehog, "Our race is marked by crimson and jet black skins. Just like yours." This states that Shadow has Black Arms DNA in him. There are several different kinds of Black Arms. Some are small and are the most commonly seen. Others are larger and harder to defeat, but carry larger weapons. These weapons look out-of-proportion on Shadow. There are also Black Hawks, which fly around in the air. Sometimes, it is required that you "tame" one and ride it. This is done by attacking it three times. There are also stronger versions of the Black Hawk, known as Black Volts. They have homing guns on their backs, and sport golden armor.
The Black Arms only appear in Shadow the Hedghog. They are the main enemy, or ally (depending on the missions you choose). There is one cut-scene where the G.U.N. Commander has a flashback to when Shadow was created. He sees Black Doom talking to Gerald Robotnik. Blaming the death of Maria and the rest of his family on Shadow, he later attempts to kill him. The deal between Black Doom and Gerald Robotnik, was that Black Doom would give some of his DNA to Gerald so that he could finish Project Shadow, but only if Gerald gave him the seven Chaos Emeralds. This would explain why Shadow has similar-colored skin.
The Black Arms appear in the Archie comics.