I think yes, imo dark web which accept Bitcoin as a payment method is a perfect place for such transactions. Anonymity and impossibility of tracking payments are features terrorists use often. For them it's a perfect way to be undetected.
Possible in this time. perfect for them because government is not included on the crypto transaction. every transactions are monitored but we don't know to whom it belongs if we don't get their wallet addresses. so they choose crypto to make an easy transaction and they can easily slip wherever they are in the world.