I did my test. I only had a black Frixion pen, so that is what I used.
I prewashed a black batik with Synthrapol in the washing machine with 3 rinses until the water ran clear.
I cut a sample of the black fabric and wrote "TEST" on it with the black Frixion pen. I thought the pen had gone dry, because I couldn't see the Frixion pen marks on the black fabric. So I tested it on white paper. The pen was fine.
Then I ironed the sample.
I was very surprised to see the word "TEST" appear in gray as I ironed. (Picture attached - hopefully)
I repeated the test on prewashed black Kona cotton. Gray marks appeared on the black Kona cotton also.
Judith