BLU can be a full job. Here's how:

[RitsukoSonoda]

Suppose Chaos dropped everyone to 1 HP at the start of the Dark Crystal phase, and you only had BLUs for healing... how would that work?

How does picking out 1 very specific obscure scenario make a valid argument when one can do so for pretty much anything in game?

I seriously doubt BLU would ever be put into a group to replace a healer due to their kit not having raise currently, status recovery, or healing buffs. You'd more likely have DPS that can fire off mass heals whenever they felt like it.