Pentel just got smart and actually developed a pen that is MEANT for fabric. This pen is awesome. Have done 5 test washes and it has not faded or run at all. The ink is as black as when I first wrote with it!
Pentel Arts Gel Roller for Fabric, 1.0mm Bold Lines, Permanent, Black Ink, Pack of 2 (BN15BP2A)
https://www.amazon.com/dp/B00AQ23VUI..._KCYkAbZZ1BWD1