To be honest the yellow tang shouldnt even be in there now, small or not. so a blue tang would be out of the question.
most tangs require a 6 foot 125g tank MINIMUM. They like to do nothing but swim back and forth all day. Putting tangs in a small tank will just stress them and they can die quite easily, especially the blue tang. tangs are the most stress ready fish i think you will find.
Hate to be so blunt, but whoever sold you the yellow shouldnt have.
Good luck and you might wanna start lookinginto a tank upgrade or rehoming, that tang is getting too large already for that tank.