What does bleeding a crystal mean anyway?
It means that a Sith takes a living Kyber crystal that has already bonded with a Jedi and forces his will onto it, and “bleeding” it in the process, turning it red. The idea is that all Sith must earn their lightsabers by killing a Jedi and stealing their crystal.
Meh.
If there needs to be a specific reason why Sith use red lightsabers and Jedi use blue/green lightsabers, why can’t it just be tradition? Perhaps in their early past, both orders used a wider variety of lightsaber colours, but they gradually stopped using such colours once they became associated with their trademark hues. Why must everything be so damn convoluted?
Sure, it doesn’t have to be so specific, and it does aesthetically limit the bad guys somewhat, but I guess they felt the colours had become somewhat too arbitrary and purely cosmetic at some point, and when they began the new EU they saw a chance to add some more ‘magic’ into the new lore. I still mostly prefer the old stories, I’m just glad to see that the new ones are putting a lot of emphasis on the fantasy aspects of SW again.