Why not. More than banking criminals will love to use bitcoin fit they illegal transaction because bitcoin is anonymous and will help them to stay hidden from the bright. Also in silk road bitcoin is highly used because it doesn't have much transaction charge, it doesn't have any barriers, it is pseudonymous and no one could ever track the transaction owner. So it's ease to use bitcoin for illegal activities.
This is not entirely right, criminals are mostly still using fiat for their illegal business bitcoin is just a small part of it, and second unless you take measures tracking your bitcoin transactions is very easy and there have been some raids that closed down some of those dark marketplaces.