I have been riding through a lot of muddy puddles, braking, and hearing grinding noises - that seems to be where the greasy deposits, er, oxidised aluminium came from!
I'm not saying that there aren't better brake blocks than the DiscoBrakes ones, but I don't think that there are many better value ones. They work well enough on clean rims so I will stick with them. The one real criticism that I have of them is that can fade a bit with heavy braking on long, twisty descents. We have some pretty severe hills round here and my brakes cope with them and they do it without wearing rims out at an unacceptable rate. For Alpine or Pyrenean descents though I
would spend more on better quality blocks.
Anyway, theory aside,
@User3094 ... spend 5 minutes with some baby wipes (or a scouring pad), and £4 for a bike's worth of blocks and see what you think!