When BLU released in FFXI, I started off using the /WHM SJ. Its usually the best SJ until you reach Lv10+ for parties in the dunes. With Headbutt (constant stunning, to lessen the amount of damage i take) mixed with cocoon (doubling my defense w/o penalty to offense) I noticed I could act just like a PLD tank, spamming pollen to generate threat/heal, and headbutt to stun and do dmg. (Later Bludgeon for large DPS)
BLU tanking worked pretty well, and using /WAR for provoke, and defender worked good enough for all level ranges.
As for BLU as a healer, it was pretty much a better version of SMN/WHM healers. You relied on the cure spells from /WHM until you gained natural heals to target other people.
(I forget what the other heal spells names were, as I only played BLU healer 3 times, at 3 different level ranges.) SMNs start to have issues healing with only cure3 around the upper Lv50 range. But at that point, you really need a solid team to level up, since monsters were no longer push overs. You needed a party who was built reasonably, and not just a bunch of random jobs.
BLU happened to get a potent heal around the Lv50 range, that was closer to that of cure4, or at least stronger than cure3. So they were better than SMNs for main healing imo.
BLU also had an A rank in its own magic (obviously) which means they could enfeeble mobs better than WHMs could. So weakening the mob enough to not kick the crap out of your tank was very helpful. Lastly, BLUs could maintain MP better than RDMs with MP Drain Kiss, and a Dark Staff.
I forget how, but there was a way to make it better than normal, but most BLUs never bothered doing it, despite the massive DPS increase it was to do it. I usually did ~85% of the parties dps. (PTs were 6 people. 3 DPS, and the other 3 were tank, healer, refresher. but i doubt the tank/healer/refresher did 1% of the parties DPS, so it was just in comparison to the other 2 DPS) BLUs either did absolutely terrible DPS, or absolutely broken DPS. It depended on the enemy type you were fighting. This MP Drain kiss method only worked on certain types of enemies(that had MP), and sometimes ppl chose to fight different mobs, so as not to compete with other groups.
Back on point, a BLU healer could go far longer with heal spamming than a normal WHM. Also if the refresher was a BRD, it was possible to debuff the enemy as the BLU, where as WHM would struggle.
BLUs biggest problem was that is was never actually great at anything. It COULD tank, but its threat was far lower than PLD RDM NIN or DRK tanks. (most ppl didnt allow RDMs or DRKs to tank, since they didnt realize how much threat they could generate) The onlys job in the game that generated less threat than a BLU were the pet jobs (excluding PUP, im not sure which could generate more, but probably BLU still)
It COULD DPS, but it was worse at ranged/magical DPS than the ranged/caster DPS. It COULD melee, but was worse at it than almost all the melee (exception being pet jobs, as some of them did worse, or roughly tied with BLU)
It COULD heal, but be worse than RDM and WHM (And eventually SCH) IDK about DNC, as I know BLU went through a lot of changes after I left, and I know DNC went through a bit as well.
The only role it couldnt do too well, was be a refresher. (I believe that has changed since i left) I did however play the role of refresher twice. (At Lv50+) because no BRD or RDM was looking for group, and both groups thought they could actually kill a monster w/o a refresher. (Both wiped on the 1st monster pull.) I switched to /BRD to give a single +1mp refresh, and focused on debuffs/buffs/heals, in order to keep the healers MP going through the fight, so we could chain monsters.
(I actually got kicked out of the 1st group, because "My DPS wasnt high enough" and they only wanted me for my DPS, despite BLUs being terrible DPS at the place they picked to grind. I couldnt possibly DPS w/o MP, and the tank always died before the mob got to half HP, since the healer was always out of MP. I switched to /BRD, and we were chaining mobs now, even if only to chain3. After that group, another group saw what I was doing, and wanted me to replace their RDM who logged out. We continued on chaining just fine, roughly 5-6, which was the average. The prior group got another DPS, and they went back to being unable to kill a single monster, and eventually broke apart.)
Anyways, most endgame groups didnt like BLU since its "Jack of all trades" just meant they had someone who wasnt good at anything. BLU always had a role it could switch to, based on the fight, to be more useful than "useless".
In omega or ultima, it was an HP/INT based caster using Breath spells. (But not in both, it was only one of those 2, i forget which. for the other, it was /WHM healer, since it couldnt DPS) In some sorta ToAU boss, it stacked defense/attack, and was essentially a melee DPS with Canonball.
In one of the dragon fights, it could only tank or heal. most groups already had a PLD or NIN tank, so they were usually asked to be a healer, despite being more useful as a tank in that fight. (Though still worse than a PLD for that fight)
I got lucky, and my dynamis group let me do any role I wanted as BLU, and I honestly changed it all the time, depending on how lazy I felt. (They preferred BLUs to be /whm healing)
In smaller fights, like story boss fights, BLU excelled at being too much of everything, and bosses in story fights had lower stats than normal mobs, but way more HP. So BLUs spells were almost never resisted, and allowed for their broken nature to really shine. They could be anything, but they were almost always DPS with a few broken support/healer/tank abilities. (Dependent on fight)
I know BLU went through a lot of changes when I left, and they raised the level cap. I heard it was more of a solo job, thats good solo, but sucks in parties, since its just weaker at everything, but got more healing/tanking/support options than it had when i left.