Kuromaku itself does not mean "evil," but supposedly, "black" was associated with "evil," and people began to use Kuromaku as the word standing for a power broker and the like, for example, "'Kuromaku' in the political community."
そのものに「悪」の意味はないが、「政界の黒幕」のように、「黒」から「悪」への連想から、いわゆる陰に隠れた人、のような用いられ方になったと考えられる。