I use the Bohin marker with the white lead for marking dark fabrics. I find it brushes off really easily. I haven't tried any of the other colors, but I suggest trying to wash it out of a sample, the same way you'd wash a quilt. If that doesn't work you can experiment with some of the stain removers like Oxyclean etc.
The important thing to remember is not to dry the quilt until you're sure the color is gone. Heat will set the stain.
For future reference, it's usually good practice to experiment with removal techniques before you mark your quilt top. I've read that the tint on some colors of chalk are created with dyes, which makes it harder to remove.
Good luck!
Janet