Which darknet sites do you know where BTC is accepted? It's been quite a while that everyone worth their salt has moved to monero.
Even this week the BTC robbers from Mount Gox had their identities exposed precisely because that coin is too difficult to laundry: https://www.justice.gov/opa/pr/russian-nationals-charged-hacking-one-cryptocurrency-exchange-and-illicitly-operating-another
Anyways, what should matter most is the censor resistant aspect. Authorities are simply unable to chase you for donating money to whatever cause you want to support.