Trying using "Alt+r" to reply. It will show you the name of the person that you are replying to. I've noticed that it's not always the last person that I got a /tell from. My guess is that the blacklisted person sent a /tell, but you didn't get it, so it looks like your friend was the last person. The server knows that you got a /tell from the RMT, so /r or "Alt+r" replies to them even though you never saw the /tell. I am just speculating this, but it seems like a sound theory.

As far as receiving /tells from someone on your blacklist; I would check the names closely. Sometimes they only change one letter in the name of new characters.