There is currently a known (not yet confirmed though?) bug where at the brief moment when you access the blacklist to check / amend it the blacklist feature doesn't work for a second or two - that's most likely what happened here as you were addidng the other spammer there at that moment. If you're being targetted by tell try to use the macro set on your action / crossbar instead to speed up the process:

/blist add <r>

This will add to blacklist last person who used tell on you.