This is semi right, and semi wrong.
Its grey enough, that you can honestly say this, and mostly be correct.
SE didint see jobs as "Tank/Healer/DPS" back in the olschool FFs before FFXI. As was explained in FFXI why they tried to not have a dedicated tank. Any tanking options, like cover, or provoke, were not meant to be "Tank" options, but were viewed as "Supportive" options. This includes most older FFs. SE saw jobs as either "Front row" or "Back row".
Using FF4 for the best example of the system (which i hated as a kid, but was actually really ingenious) you had 5 slots, and it was 2 back + 3 front, or 3 back + 2 front.
This means at LEAST two people HAD to be in the front row, which means 2-3 people HAD to take double damage. THFs had evasion, MNKs had HP, PLDs had defense, etc.
BLM had no defenses, but was able to do maximum damage from the back row. Front row users "usually" had less effective options for the back row. And of course back row users were never hampered by the back row.
Healer vs Ranged(physical or Magical) wasnt quite established honestly, because of various factors, such as WHMs being rather strong with Hammers in early FFs, but Rosa was the 1st to not really melee. Her Holy was strong, but it took a long time to get her to use Holy. But FF4 was the 1st FF were u couldnt just stack WHMs, so she didnt need alternative options. Every job you would consider a "Healer" had good offensive options at some point. FFs were you could pick to add a "healer" (WHM usually) never actually needed a "Healer".
Of course a lot of this starts to change by FF10, as provoke was finally required for a single fight. Yunas WHM healing still was never required in any of the fights. (She was still better as a DPS to zerg bosses down)
So, looking at how SE saw it as "front row" vs "back row", its better to say the "Jack of all Trades" jobs were those who could be both a front row, and a back row job, and have little to no performance issues. BUT, so to be clear, I said that to give a generalized impression. Obviously having heals would be the secondary requirement to meet "jack of all trades".
Defensive roles pretty much just meant "front row". Not how well they managed to protect allies. Almost all healers were good at DPS, with a few exceptions in the series, such as FF4 Rosa, who could only be a DPS in certain scenarios. So its roughly safe to suggest there is a "healer" role, as a few FFs did have the role, but most didnt.
RDM and BLU were capable in both the front and back rows, and had heals. (along with defensive buffs for themselves or allies) So IMO, being a "DPS main" applies to all jobs. But being "multi-role" is definitely in their design. This is why I said earlier you were being rather misleading by saying BLU is a DPS main, with supportive abilities. That pretty much applies to every job in the series, based on game design. THFs "Steal" is literally a way for you to get powerful gear early, which makes up for THFs lower damage. Instead fo a skill that buffs a party members stats, its a skill that lets u get a perma "buff" (gear) MNKs bonus of not needing gear helps u save gil for the members who need the money more. (Depends on the game, but FF1 is the best example, where gear is costly, and magic is even more costly) Not all supportive options are skills you directly use. (Like with the MNK example) Every job has a design philosophy behind it.
Now to help support your argument, The devs have repeatedly seen BLU as different things through the series, but with examples of FF6, and FF11, the DEVs saw BLU as a Caster 1st and foremost (Back row only. Even FF14 gave RDM a melee combo, because they see it as multi-row still). Which I feel is how they saw it in 5, despite its ability to be "multi-row". The only reason it was "Multi-row" was because it started off with no spells, and they also didnt know which spells you would find, if any. In FF6, Strago not only starts with blu magic, he can also equip an Esper immediately, and have access to white and black magic + a summon. So there was no reason to give him front row potential.
So BLU is only "Multi-row" due to necessity, dependent on the game. RDM on the other hand, was clearly intended to be "multi-row" in FF5, as well as Versatile from FF1.
(This is why i actually saw RDM as the better "Multi-role" over BLU, despite BLU being a great candidate for it as well. I also felt BLU would been good as a "Choose your DPS playstyle" picking between Ranged vs Melee, and Support DPS vs Selfish DPS, back when that was a thing...This is of course just opinion, as theres more to deciding which job becomes a tank besides just being "multi-row" or even versatile.)
EDIT: Rosa technically had 11 "offensive" spells, 13 defensive spells, and for spells that could be used as either offensive or defensive she had 9 (example being how many undead littered the game, and cures can hurt undead, but melee attacks barely touched undead) I also listed cures1-4 as 4 different spells, despite being essentially the same spell, and raise/arise as different spells. If you dont consider them different spells, then she only has 10 defensive spells vs her 11 offensive spells, and by a statement you made about the number of offensive spells a person has, is how you determined if they were pure DPS, or DPS with support. This is another reason why I say this logic is misleading, even if it could be true.
EDIT#2: Interesting side notes, BLU magic in FF5 wasnt truly considered magic. (most likely due to having physical abilities and spells in the same spell list) So things like silence, and dual cast dont work with it. But this was probably due to engine limitations mostly. Maybe. For BLU spells that were magical, like Aero, all magical spells defaulted to the BLU mages magic stat, even if you were using BLU magic on a non magical job, such as a Knight with BLU magic equipped. This sort of acts like the polar opposite of RDM. Where physical jobs could use blue magic w/o penalty, but dual cast from RDM was pretty much only useful as a caster trait.