Coincidentally, I just rebuilt a tank using a fresh bag of Flourite Black. If manganese leaching occurs, I figured brand new and dusty substrate should release an initial burst. So I tried your paper towel experiment:
The characteristic purple shade is almost certainly proof of manganese. I left it in for two days, and forgot to check on it in the meantime; but if I had, I'm pretty sure it would have been a lighter shade of pink.