Nick I've used the camo PowerPro for years and have never seen or heard of the color coming off, in fishing applications that stuff will cut your fingers though.
Also...
I use the 130# 'DACRON TROLLING LINE' (second product down) for our down riggers, it does come in black and I've never seen nor heard of the color rubbing off either. This is strong stuff, earlier in the year we were trolling for grouper on a shallow reef with the down riggers and I was running one of the weights a little low and caught the reef, rather than breaking the Dacron it twisted the down rigger and tore the built-in rod holder out of the boat. I believe it would be perfect for your application.
Dacron Trolling Line
