If its turning black, buy a new seal and replace it, then get some silicone grease (made for o-rings, can be bought from plumbers/dive shops/pool shops etc and smother the inside of the slave with it. The fluid turns black becuase the seal wears away from lack of lubrication, the small black particles pollute the fluid and this wear also causes premature failure of the seal. There have been many success stories using this method, I have done it on my recent bike and the fluid has been clear and clutch working flawlessly for 2000 miles.