Are you speaking on the US or worldwide? A quick search doesn't produce any reliable statistics that indicate this. I have never once heard of anyone purchasing alcohol on the black market. Cigarettes, sure, but even then it's typically only very low-income folks purchasing cheap, illegal tobacco that is generally not of great quality. This is in stark contrast to marijuana where high-grade product can be purchased on the "street".
I suspect that once(if) pot goes legal the black-market will consist mostly of cheap, poor quality marijuana. This should take a nice chunk out of the profits that gangs see from marijuana distribution.