If the fluid really doesn't cycle then why is it black before much use? Just a thought when in the saddle for too many miles!

A) the fluid is hygroscopic, meaning it will absorb moisture, hence the sealed cap, and
B) Brake fluid will remove oxidized rubber (and mold release) from the internal system seals. Wanna see black tires, rub a rag dampened with brake fluid over a tire side wall. The tire will be "like new" and the proof of my above statement will be left on the rag.