Blue Mage as a full job wouldn't really be Blue Mage. As it would need to have a set selection of spells they learn as they level, no learning spells from enemies or anything, they would be just a minor variation of Black Mage.

The Limited Job system was more designed for them to just go silly with ideas that don't need to be balanced or anything. They can have classes that can completely exist outside of the "meta".
I do however think they should allow Blue Mage to reach the regular max level, but keep the restriction that they can't enter random groups with the Duty Finder and to stop people from cheesing Raids and Trials with BLUs make them not drop any loot if there are any BLUs in the party.