I was given a topper to quilt,once that had blue marks on it from when my friend was going to quilt it herself and changed her mind.(I think what she had done was ironed over it and set it) After I was done quilting it, the marks she had put on would not come off. I asked my mother what she thought. She took it home with her and soaked it in Murphys Oil Soap. She also used Oxy-clean. Don't let the name scare you.It cretainly did me!!I thought for sure it would leave a greasy or oily something on it. It didn't But after many times of using a small brush and scrubbing it, the blue did come out. Much to my friends happiness.
I have one that I used yellow chalk on white fabric,(I don't use yellow anymore & have never used the blue pen for this reason) to mark the design and it won't come out but am going to try the Murphy's Oil Soap. I hope this helps you too.
The bright blue "was" in two of the center white squares at the top