Maybe the difference is that her hair is black but he had it dyed.
El tiene el pelo azul ahora. Antes lo tenía negro.
I'd never say "él tiene pelo azul" because it's impossible to be born with blue hair. That's why I'd say: "El tiene el pelo azul. Se lo tiñó".
Hope it helps you.
I don't think that's the reason because, if that's the case, we can say:
"Este joven tiene pelo azul. Esta joven tiene el pelo negro.", but it doesn't sound natural at all.
Just want to add something that may help.
Would you say "El tiene pelo azul" or "El tiene el pelo azul"? I'd say "El tiene el pelo azul" referring to the momento of speaking. Maybe tomorrow "El tiene el pelo colorado". Pero no nació con el pelo azul ni colorado.
Actually, from a strictly grammatical point of view, both are correct. You CAN say "él tiene pelo azul" or "yo tengo pelo negro". No need to use the article. In practice, using the article sounds more of description and not using it sounds like a statement... kinda like saying "I wear my hear short" instead of saying "I have short hair".
Can you also explain the rule for using an article in front of a body part. I thought normally you would use the article. Like- La niña se cepilla el pelo. Ellos se lavan lasmanos con jabón. If you have now of an article that explains this well please let me know. I searched but couldn´t find one.