I find this is a bit of a difficult and touchy subject - in a case like yours, where fine-tuning seems to be the main issue, I would personally deem it more appropriate to offer to give advice instead of just blasting it out aswell, but since you made more of a general statment here, I would like to call that into question.

First: If someone totally fails at the basics of their job, specially a tank or healer, specially in content past Ifrit NM, I dont see the need to politely ask if I'm allowed to give them the most basic advice. I will not ask if I may tell the gladiator that they need to use Flash. (And yes, I know, that it wasnt about the most basic-concept of BLM in your case, but if you make such a general statment, I'd like to show when it doesnt or shouldnt hold true anymore).

Second: Even beyond the most basic advice, I dont see the need to beg people to let me tell them that they're getting certain, very specific things (like AoE-spells vs. non-AoE-spells and when to use which) wrong. Its a waste of everyones time if all you want to do is let the person in question know that Thunder IV isnt stronger than Thunder III when used on a single target. (And yes, I know, in your case it was followed by a whole lecture, from the sound of the OP, but again: general statment and such).

Third: If you are already aware that you're struggeling with a job, I personally always found it very useful to let the party know before hand real quick that I'm rusty on said job and still learning. Specially in past level 50 or even more so level 60 content. And yes, you can obviously say "I'm in a leveling dungeon, it should be obvious that I'm still learning!" and it should be infact obvious, but I always wanted to give my fellow team-mates the courtesy to let them know that I'm fully aware of not being a geart ninja and that any advice would be appriated.
Thats a bit of an issue that I'm having with your statment there - you are aware that you're not a great BLM (yet) and thats obviously not a problem!
However, I've always believed that you shouldnt have to beg people to be allowed to tell them things they should already know or be able to do. Advice should be given in an appropriate and polite manner, but I've seen so many people out there who believe they dont need to hear or follow advice that the overall skill of the playerbase would decrease even more if it were up to them to decide wether or not they need it (surprise: they think they dont and believe they play like young gods already - anecdotal, but to illustrate: Last week I met a level 50+ BLM in KoL, who didnt use Fire II on groups of 4+ mobs, because he felt Fire I was stronger. Even after being told otherwise.)

I will not beg to give (fundamental) advice.
If you have an issue with advice given to you, I can understand that - its annoying, specially if you actually know better and are already busy beating yourself up about the mistake you just made. But personally I always found it more helpful for myself and the groups I ran into, to inform that I'm not terribly familiar with the job in question, but are trying to figure things out and that advice would be welcome.

I'm getting the impression that a lot of people - maybe you aswell, OP - think of advice as a personal attack, which makes it really difficult and annoying to give out any advice or even offer advice. Even by asking "Hey, do you mind me giving some advice on your rotation?", you're implying that there is an issue with said rotation and I've seen a lot of people taking offense at the suggestion that they might not play like the young gods they think they are.
What needs to change here is not only the way that some advice is given (advice should always be given in a polite manner), but also the way advice is recived (not as a personal attack, but as feedback that the game itself gives terribly little of by itself).

Its not that I dont understand what you mean, but I'm saying there are other ways to deal with this problem - mainly to inform your party that you're learning/struggeling with the job or are still new to it/rusty.
If you absolutly dont want advice, thats the least you can do - besides the obvious going into a dungeon, knowing how to play.