Too bad this has nothing to do with actual gameplay benefits of that decision. There is pretty much never a gameplay reason to blind or castrate someone. In my opinion it should give some bonus to relations with one's vassals, reflecting that they would think twice before they cross such a ruthless ruler. Similar to "crushed a major revolt" really.
Blinding/castrating enemy dukes/kings who I have caught during war shouldn't cause any problem to me. Apart from them hating me. Doing that stuff to your vassals might cause problems.