Never heard of that... Was it because you could buy illegal items on FB marketplace?
Typically black refers to the absence of light. I think dark is a better term personally than black. Dark pools are clandestine exchanges where you can buy and sell stocks or coins without government oversight. I refer to the labs in China that make our peptides as "dark labs", since there is little to no government or regulatory oversight or certifications. And black lists are lists you don't want to be on.
But if FB Marketplace was called the black market back in the day, I'd suspect you could find drugs, cable descramblers (oh the good ole days), flesh, and other things that were against the law.