Bitcoin is insanely volatile, but fundamentally works on same principles as any other currency does. Albeit, it is not regulated, hence it's volatility.
A currency's fluctuations in value broadly depend on the strength of the economy it represents. As the economy grows, the value/purchasing power of its currency does too.
So, what about the black market? All the trade that happens, either illicitly and in direct breach of laws, or just outside of the scope of those laws but not technically illegal?
I believe the rise in value of all these currencies like Bitcoin and other cryptocurrencies, in some way represents the value of these 'hidden markets' and hidden economies. It's difficult to prove, because these markets aren't subject to the same monitoring and measurement by our governments.
There's a slight irony in it, that I know some fairly left-leaning people who are into bitcoin and such things, because they apparently are against our current economic systems, and generally advocate for more government control of private enterprise, whilst simultaneously buying into currencies that represent the most 'free market' type economy that has ever existed on such a scale. It doesn't sound like all their cogs are spinning the same direction.