The Black Mamba was a cape made for Megamind created by Minion before their quarrel. It was wore on his back and was high-collared with blue at the back and spikes on the back of its look.
When Megamind's cape was normal, it had no tears on it.
As Metro Man in The Button of Doom
Although Megamind also had Metro Man's white cape in Megamind: The Button of Doom, he didn't need to be a hero and wore it along with his suit.