I read about this rear brake fluid blackening elsewhere on here some months ago (I don't have time to find the thread to link it). Anyway, it definitely seems "normal" for our bikes.It's common on these bikes that the rear brake fluid turn black fairly quickly. Seems to be from the heat. I'm at 4,350 or so and have replaced my fluid twice. My clutch and front brake have darkened a bit but not too much.
I run the Motul RBF600 in my bike and my car, it's good stuff.
One caveat though: Racing fluid like this needs to be changed MORE often than standard brake fluid, so don't think that putting in higher quality fluid means you can go longer between fluid changes, it's the opposite.