There are lots of metrics in favour of Bitcoin like the one in Forbes that said the black market is worth 10 trillion dollars and they need a global currency or if only 1% of global transactions went though Bitcoin each coin would be worth ~$64,000.
What makes you think that all the people on the black market would switch to bitcoins, if only people doing illegal things used bitcoins? The only entry points into BTC are the exchanges or mining, and if 90% of btc traffic was illegal stuff, the exchanges at least would be under massive scrutiny.
Isn't it also far more advantageous to have a currency you can actually spend
in useful ways? If all you could get by selling weapons was btc, and you could only buy other weapons for btc...would anyone bother with btc? There's no non-weapon profit to be had, whereas there is tons if you go via USD.
It doesn't make sense to expect or rely on the idea that an exclusive use of bitcoins by the black market would make a bitcoin extremely valuable. If everyone in the world accepted bitcoins for goods/services, then
they might add demand if they can use it almost anonymously.
Have you recently tried to send money to an individual in another country? The form is bigger than applying for a passport. Two proofs of ID, proof of address and so on,,, and your not even asking for a loan, your just trying to send $100 of your own money to somebody.
"black market" probably loses about 20% of its revenue just moving money around.
Because the banksters won't allow good things to happen first with bitcoin.
The fact is its still quite hard to buy bitcoins without having to wire money and wait a couple of days for funds to clear.