Blue Magic has to be the most useful in an overall sense; it takes elements of both Black and White Magic, while often adding creative twists. It can also, in certain circumstances, overwhelm an entire body of White of Black Magic with a few spells. In FFVII, for example, White Wind, Angel Whisper, and Big Guard are undoubtedly more useful than all of the typical White Magics combined. Normal Black Magic in FFX-2 consists entirely of elemental spells; something like Supernova or Annihilator is clearly more destructive.

Blue Magic also has twice occurred as a Limit Break set for a character. You can't tell me that Shockwave Pulsar and Degenerator (FFVIII) or Nova (FFX) are weaker than the Black spells in those games.