Black Belts turn into Masters. It IS possible to beat the game with four White Mages. They had an NP Arena Challenge about it awhile back. I once took up their FF3 challenge of beating the game with all the characters as imps, and I did it! It wasn't really that difficult, though, since their levels were all so high in the save file I used.

"Prepare for trouble! Make it double!" - James FP