That's the point. Magic draws energy OUTSIDE the body. Qi draws energy from the inside. Tatoos are external. So the tatoos strengthened Negi's draw of magic. For Chao it allowed her to draw the fire element to use magic herself. Black is the combination of all colors so it has the most draw. That was the point of the spell "To consume all power".
Also in addition, you can use your original specialty, in which Negi's case is lightning, thus he lights up.
Evangeline is ice but she doesn't need it anymore because of the risk and her current power