Two possible fixes:

Enable the ability to blacklist someone who has sent you a friend request, without declining the friend request first. (Or, make the act of blacklising auto-decline the friend request.)

Enable the ability to blacklist someone who is offline. (Just in case the spambot tried to dodge your blacklist by logging off while you fumbled with the friend request.)

Honestly, both of these should have been in game already. Why shouldn't you be able to blacklist someone who's not online? Why shouldn't you be able to blacklist someone who has send you a friend request?