I love the Frixion pens!!! I am using them on my 3rd quilt. I have heeded the words that some say the pen may discolor on some fabrics, so i always test it out. I'm not worried about "leaving" the ink in the fabric because I always wash the quilt prior to giving it to the person. There are also reports that the ink reappears if the quilt is in temps lower than freezing....but that doesn't happen for me either.
I hope those who say they are worried also don't use the disappearing inks, because theoretically, wouldn't those inks still be there?
I am struggling to mark some dark fabric and I'm going to use some ivory soap. The other thing I often do w/ dark fabrics is keep my quilting lines straight where I can use masking tape to mark my quilting.