you're harping on the wrong thing, BLU's problem isn't that it can be played in "old content", BLU's problem is that it CAN NOT be played in CURRENT content.

NIN, DRK, AST, MCH, SAM, RDM all could be played in Halatali during release. the problem with BLU is that you can't play it in Ivalice or the MSQ that's going to be released in the next patch.