I don't understand why Blue Mage's status as a Limited Job would extend to the Trust system. It doesn't cause trouble for anyone except the player taking it on by being over/underpowered. The AI companions can behave identically to if you were a Black Mage - I'm fine with being considered Spell DPS even if I have Aethereal Mimicry.
Beyond the first few weeks after a new Blue Mage release, the worst thing about BLU is finding people to go to dungeons with you. Being able to bypass that - even if you were only releasing "Blue Mage can go into the next set of dungeons" one patch later - would be a way for players who come into the whole thing later to experience the content without fellow players who've already done it (sometimes several times, due to the original spell gain system) having to do it over again.
The biggest part of it that baffles me is that you've already got the Trust system programmed, and don't have to do anything extra to let the Blue Mage engage with it.