User blog comment:Staw-Hat Luffy/Dat Blind Guy/@comment-5896530-20130306141840/@comment-4530178-20130306155102

Jimbei mentioned that the World Government labelled Blackbeard as a Yonko in the Fishman Island arc some time during the two years time skip. Based on this we can assume that its the World Government that give out the titles of the Yonkos.