With all the mixed information out about the Frixion pens, I wanted to get the definitive answer for myself. I used my black Frixion pen to mark the pattern on a lap quilt for my Redwork. The fabric was starched and ironed before cutting out the blocks but not washed. I drew the pattern (a couple of times before I got it right) did the Redwork, constructed, quilted and ironed the quilt to remove the black marks.
I washed my quilt in the regular top load washing machine on gentle with a sprinkle of Tide and fabric softener in the dispenser. I ran it through a brief wash/rinse cycle. I put the lap quilt in the dryer to almost dry and blocked it to finish drying. After it was completely dry, I loosely folded it and put it in a large bag with the top open. I put my bag/quilt in the chest freezer for 3 hours. The Frixion marks are just BARELY visible. If you didn't know they were there you probably wouldn't see them. I will throw the quilt in the dryer today to remove the faint lines.
I will be using my Frixion pens again as I need them. If I ever (not going to happen)was sending a quilt to a major competition in a airplane cargo hold(very cold) I might be concerned that a judge might notice the faint lines while judging.