Try some retro clean

What a beautiful quilt! I am sorry this has happened to your beautiful work and I hope one of the suggestions will do the trick of removal.

I used club soda on blue marking that had been ironed and it came out. I had not washed the item when I got the blue out. Good luck.

Another idea....my daughter totally believes in the Tide pens. I have never tried it on something like this, but if nothing else works, maybe it is worth a try.

Is it the outside corner? if so I would remove that corner square and pop either the same design back in or something different

I've tried just about everything everyone suggested & so far no luck. I have it soaking in vinegar water right now.

I was really hoping the Blue Line Eraser would work. Tried it several times again & no luck with that either. It looks like I'm stuck with it. Lesson learned. Do NOT iron before removing blue lines!!

>>Is it the outside corner? if so I would remove that corner square and pop either the same design back in or something different <<
It's all 8 of the mitered corners in the borders. I'd have to rip out the binding, all of the borders & many embroidered designs. Many of the embrodered designs are 15 inches long. It's just not possible. I can't remove them & do them over.

I have kicked myself many times over this :(.

I don't have any better suggestions than those already offered but that is a beautiful quilt - the only thing I can think off if the marks remain is to applique over it or some other embellishment?

Salt? a salt solution might be worth experimenting - though i agree with others ... test on scraps first and only treat the quilt once you find something that works.

I am so sorry that your blue line didn't come out. I can understand how you must feel, as I have done the very same thing & didn't realize it until it was to late to fix. Your work is lovey though & you should be proud of that!!
:D :D :D :D

Crazy thought - Have you tried using the blue marker? Get a scrap and heat set blue marker on it then go over the same area again with the blue marker and see if it removes it. I know coffee removes coffee and wine removes wine. What can you lose at this point?

You could couch a cotton yarn over the seam, it would go thru your embroidery, but if done on all the lines it will become a design element. Other yarn alternatives would be thin ribbon, or turned fabric tube, like for a spaghetti strap, made from same background fabric. I think the tube from the same fabric would be the least noticeable.
This is how I make mine. Quilting cotton is heavier than the fashion fabric she is using, so you will have to make the tube a bit wider. Working with shorter lengths is much easier than trying to turn a single tube for the whole project.
http://www.youtube.com/watch?v=muPk3tXV4I0