A couple of things, Your fluid is not getting 'burnt' by heat, brake and clutch fluid can withstand way more heat than the engine can stand, your engine would probably seize before you 'cooked' the fluid. I've seen disk brakes on race cars glowing red at the track. brakes still work.
As Lee said, the black is a result of black rubber components, seals can wear and not leak, the line from reservoir to slave is black rubber, etc. etc.
Just the tiniest amount turns the fluid black.
'Is an aftermarket slave better?'
The stock slave is aluminum, normal, slight wear in piston and seals produces black residue, (wet sand a piece of aluminum, the water goes black.)
Some say the fluid lasts longer with aftermarket slave cylinder, that's because they use anodised aluminum, does not produce black residue with normal wear and have better quality seals etc. Vitron seals/o'rings are usually green or blue.
Clutches have a full range of movement which pushes the fluid back up into reservoir, unlike brakes which have tiny range of movement. Hence the clutch fluid mixes more than brakes.
The 1200 clutch design uses a sectioned pushrod to prevent spinning against slave, but as said, still not enough heat to burn the fluid.
It's actually quite normal for clutch fluid on a bike to go black quickly and nothing to really worry about, except of course, it looks bad.
My clutch fluid has been black for a couple of years with no bad effects, might get inspired to change it soon.
