Plain old cash, including western union, money gram, gift cards and even PayPal.
Bitcoin is used more by "bored" western whales and by the Chinese that need to put their money into something the government can't control than by drug or arms dealers.
What Darkmarkets still use cash, western union, PayPal, etc? My understanding is this all died out once bitcoin became a viable currency.
Then again, I guess I'm mostly just thinking of drug-focused markets, not the other kinds... but still. It'd be bizarre if those kept using the payment methods you mentioned.
Typically Bitcoin-only, but in the past 1-2yrs Monero (XMR) has gained acceptance in certain large venues. I'm not sure, but I suspect your parent commenter may be referring to the large amount of darknet commerce that does not take place using cryptocurrency?