BLU is a new piece of content akin to DoL/DoH, who also can't participate in the MSQ. This may change in the future, but for now we need to let BLU have a teething period before the devs can open it up.

Put it this way, these are the same devs that thought that resetting cooldowns after a wipe would break the game, so having them limit BLU from content for now, only to reverse the decision later down the line after the playerbase has road tested the job, isn't all that surprising.