Look up "Silk Road". It's illegal if what you're buying is illegal. Some of the shops (like Silk Road) only take Bitcoin.
Out of curiosity Ive made a little search for it and it's close now but still transactions there are still continue maybe in other way. I just wondered what items or things that those 5 voters are purchasing there with their bitcoins.
Do not use your bitcoins in Dark Web. Do not feed the illegal network with bitcoins. This activity damage it. Is this use of bitcoin that make that most of the important people that make decisions in our countries define bitcoin as dirty currency.
Definitely will not use it for those purposes. Besides I don't know how to access those deep websites even Google tell me how to do. It's just that I don't want to try even my curiosity always asking me to have a look at that world lol.