I had to replace my slave cyl a couple of weeks ago. It could have been worse, I was carrying a new Oberon and I was in Gosport so I rode to the local KTM dealer Trevor Pope and bought oil etc.
The experienced mechanic there popped out to see what I was doing and loaned some advice and bleeding eqpt. The first thing he checked was the Master cyl. Looking for signs of air getting in as the clutch lever is released.
The last thing he checked was the slave cyl and indeed this was where the oil was dissapearing.
For bleeding...his method was attach a pipe and catch-bottle to the slave bleed nipple. Fill the Master res. Open the bleed. Stand back and admire the handsome lines of the 10 year old beast and let gravity do it's thing. Keep topping up. Keep admiring. If it makes you feel better then do all the fancy moves and butten it up.
People talk about moisture boiling, oil boiling etc. I can't imagine the oil getting all that hot. I must remember to put my hand on the pipe/Master cyl next time I ride to test it but I'd be very surprised if it was hot. Any black deposits in the res would be normal wear debris from the Master seal/bore IMHO.
If you are keeping the slave 'o' ring in good shape then perhaps the fault is at the Master seal?