When does Blockchain.info show 127.0.0.1 as "first relayed by" for a transaction? Does this mean the transaction had to come from a Blockchain.info/wallet user (or perhaps someone using Blockchain.info/pushtx)?  Or could this indicate something else (e.g., a node from Tor even)?
Yes, It means blockchain.info broadcast the transaction itself. So it was either created using My Wallet, the merchant API or submitted through /pushtx. A live list can be seen here http://blockchain.info/ip-address/127.0.0.1

Mixer transactions are broadcast through separate nodes.