Only advice i can give is ... delete both off both lists then readd to which ever you want them on to 

 

To delete click on the buddies or blacklist tab then look far right you will see a * sort of icon click that then look for the player name and click on the bin icon to delete them off the list.

 

If you cant delete contact devs using the "REPORT A BUG" and let them know, but ive never seen that issue before myself