It's like comparing SAM vs melee RDM in terms of damage output. Of course SAM will always win, but does that mean RDM is a useless job?
BLU is good at everything, but not awesome at anything.
You can attempt to gear the job towards a specific role (and most of the time, people do that in order to replace a melee DD), but as you've probably seen, it won't work out too well if you try to turn BLU into a pseudo-copy of another job.
But still, BLU has so many strong points to differientate it from other jobs it's not even funny.
It's the only job that can deal consistent physical and magic damage (PLD, COR, SMN and PUP can too, but they all have recast/TP/pet limitations, which are worse than BLU's MP ones), has one of the most enmity generating (if not the most) abilities in the game in Diffusion+Exuviation, a ridicolously overpowered damage+superstrong Bind spell in Regurgitation, the shortest recast stun ability in the game that doesn't require a 2h (although not always reliable).... this is just scratching the surface of the job's potential.
The job loses effectiveness in large scale events where you're more likely to have more specialized jobs to cover a certain role (and even then, it has a niche in Cannonball in those cases), but otherwise BLU is almost always a solid choice (except Arrapago Remnants since without cells it's fairly useless) as long as you play it by its strenghts (read: BLUs that can only spam the big three and pretend to be top DDs everywhere can gtfo).
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