Together, 幕后 (mùhòu) means “behind the curtain” or “behind the scenes.” 黑手 (hēishǒu) means “black hand,” a term for a sinister, hidden influence. The combination paints a vivid picture of a hidden, evil hand manipulating public events from the shadows.
The connotation is always negative and serious. Accusing someone of being a `幕后黑手` is a very strong allegation of malicious, secretive manipulation.