I think folks are about as likely to fill in the “illegal gambling and drug transactions” part their tax form as they are to remember to report the stuff they bought online.
Honestly, as much as people shit on Bitcoin for facilitating black market transactions, I think that these are by far the most interesting. Silk Road was a pretty solid example of how you can use Bitcoin to bring trust and a little safety (from drug dealers) into black market transactions. By all accounts, getting drugs on Silk Road was a pretty sure what to make sure that you get good properly made drugs. Outside of law enforcement, the biggest danger in a black market is a lack of trust. A lack of trust is what gets people shot or given fatally flawed drugs. Bitcoin, for a brief time, offered up a pretty appealing alternative.
In an ideal world, we wouldn’t need a black market because people wouldn’t have to resort to it. We don’t live in an ideal world. I would rather live in a world with a safer black market than a world with a dangerous one. Attempts to keep shut down black markets do vastly more damage than the actual products being sold, especially when there is a little trust within the system.