Mine sweat too when the humidity is very high, I suspect we have the same setup. Only noticed it a couple of times when it was near 100% humidity. I have changed brake fluid several times this summer (uprated rear brakes/bias valve/Pagid pads/race master cylinder) and have not noticed any black flakes. As long as you have fluid in the reservoir and the fluid is not contaminated it should be safe, right?
I'm sure you have thoroughly researched the best hoses...please share your thoughts.