Dev -- the singer behind the smash hit "In the Dark" -- is desperately trying to block her sex tape from going public ... claiming the mystery seller is just "someone trying to embarrass me." We broke the story ... the footage is being peddled to various media outlets ... showing Dev with an…

"Dancing in the Dark" singer Dev -- also known as the voice behind "Like a G6" -- is front and center in a brand new sex tape, or so it appears ... and TMZ has learned it's currently being peddled to the highest bidder. Sources tell TMZ, the tape was shot last year with an ex-boyfriend -- and it…