I think the crystal become black because he able to use all attributes, the same way that when you mix all color to together and it will become black. I'm sure i saw him mention something like that in some chapter and i remember that because it make sense.Of course the blood parents blame him and wants him back after he reveals his power, classic trope
Even though they are the one throw him off because his appraisal shows dark magic as his majority power hence the crystal become black
I think you're misreading that scene. Kyle's bio-dad hasn't come to suck up to him and take him back (yet), page 21 is the bio-dad looking around the infirmary for Leonard + yelling at him. He's not saying "how dare you (Kyle) hide this great power from me, I want you back", he's saying "how dare you (Leonard) conceal your shameful dark magic (which Kyle go him to reveal) from me for so long", because as a family they think dark magic is abhorrent and he thinks it's shameful that Leonard's revealed himself as a dark mage.Of course the blood parents blame him and wants him back after he reveals his power, classic trope
Even though they are the one throw him off because his appraisal shows dark magic as his majority power hence the crystal become black