I've managed to level my chocobo from rank 4 to rank 9 just leveling my BLU. I'm going to keep grinding it out until they nerf it.

Interesting if the real reason has to do with them assuming it would be Level 70s powerleveling blue mages or groups of blue mages together with their chocobos or single blue mages with chocobos... and so they just gave the exp boost to the entire party rather than coding it specifically for blues and chocobos only. Hm.