Article link:
https://www.wired.com/story/bitcoin-drug-deals-silk-road-blockchain/

One of the techniques mentioned was looking through the vast BitcoinTalk threads and finding those with wallet addresses on their accounts.
While that address may not be the one doing the transaction, the two wallets could show links in the blockchain and have repeated small transactions as money is moved from one to another.  It seems like an interest case of analyzing an extended communication network to uncover patterns (some the folks at FBI and NSA do on a regular basis).