I dampen a corner of a white washcloth with cold water and dab on the blue line.

any other thoughts before I throw it out?

a very weak solution of chlorine( bleach) will solve that problem use it on a cotton bud to follow the line. dab with a damp cloth straight after to remove the bleach.

I never use these blue markers because I have been dissapointed too. They either need a complete dunk in water or they stay in the fabric as a brown faint smudge. I heard too that the chemicals in the pen can cause a weakness in the fabric threads.
Now I will always stick masking tape and mark on that, The items then don't require any stress and can be ironed.
Happy Stitching!

Have you ironed it,if so you have made it permanent.
I have done this several times.

Is the erasable end dry? I think they use hydrogen peroxide. (Hair bleach)

I always get mine out no trouble with water. I rub it with my wet finger till it is gone. Need several dips in the water to get it out and rub again etc. than I let it dry

Have you tried a dryer sheet lightly rubbing the mark. I have had success with them. Depending of course on the type of fabric you are using.

Ann