Blue mage can participate in Battle content. Just not current content.
So basically, it's a minigame that utilizes past content.

People, correct me if I'm wrong here... but MMORPGs ultimately boil down to three systems: Leveling, minigames, and endgames. Right? Anything that doesn't contribute to the current tier of progression (endgame) is a minigame. Sure, there are a few "but what abouts!" to consider, but generally speaking... if it's not endgame, it's a minigame. If you're not first, you're last.

Not trying to hate on Blu here, but let's keep it real. If the bulk of Blu's gameplay is in obsolete content and a single arena where it plays solo, then that's not endgame, which means it's a minigame, and to try to call a minigame "battle content" is deceiving. That's like saying FFXIV has sports content because of that silly Golden Saucer minigame where you kind of shoot baskets.

Also, is ANY of FFXIV's minigame content actually popular? Other than housing, which is still only marginally "popular" because so few people can actually own houses?