I am guessing that your actually ideology isn't as hardline as the question you asked would imply. But just incase it is; consider a country like North Korea, certainly the black market there provides a lot of social utility.

There's also a good argument that Silk Road provided social utility in the US because anonymous transactions over the internet are less prone to shootouts than the physical black-market.

If you're looking at it from a harm-reduction perspective, people are obviously getting drugs despite prohibition, so you might as well have a less violent drug trade.