Hard to say what's the biggest shadow market of course but afaik WHM is the largest one for drugs and they only accept Monero. You're right, Bitcoin is still popular in the Russian forums, but that's at least partly because it's easier to cash it out anonymously over there. Lots of services that let you exchange your BTC for cash and my guess is the remaining coins just go through tumblers until they're clean.

https://news.bitcoin.com/darknet-giant-white-house-market-dr...