I'm back again - all I seem to do is post in the mechanics section of this forum

Anyways, I'm taking my bike into the shop soon and something reminded me about cleaning brake discs. I've ridden about 550 miles on my bike and have never cleaned my brake discs. The colour difference is quite noticeable from the (no idea what the technical vocab here is) stem type things that attach it to the hub, and the part where the actual pads touch.

It's a sort of black-ey colour where the pads touch and on the stem-ey things it's just a normal stainless steel colour.

So, my question is, how often should I clean them? Is the black-ey stuff going to do anything to the braking performance? Is this a sign of something happening with the pads? And if I need to clean them, what do I do?

Thanks for any replies,

Buggington