Playing as a mage I have noticed my hands have become scarred and darker since the start of the game.

Impressive to see but does anyone know why this has happened? And can the hands be restored to how they are at the start of the game? Is it to do with disease? Or something else?

I've never experienced this... actually, I've never noticed this. I rarely look at my characters' hands.

My mages mostly wear gloves for protection, so never noticed this. EXCEPT, you do get charring and effects on your hands when you cast spells... dunno if this is what you mean?

Using first person view you get a good view of the hands when using magic.

Never had this happen to me, but my Hands are usually covered in Fur anyway ._.

No it is permanent hand disfigurement. Not just something you see when using spells.

Never noticed it on my mage, since he always wears gloves, nor on my witch, since her paws are fluffy.

That's odd, I've never seen this effect, and I'm running a mage character myself. Her hands look exactly like they did at the beginning of the game, and I've never noticed her hands becoming "charred" or anything like that when casting fire or shock spells, for example. If she's injured, sometimes her hands will have blood on them, but that goes away eventually.