My youngest was completely blonde when he was younger. He, now is a light to medium brown.
HOWEVER - I noticed the same thing with MJ's oldest son. And then I looked closer and he had out growth. My son's blonde was blonde... it didnt suddenly stop and have root's showing.
Look: (I'm gonna look for a picture with 'roots' - I saw it on a video)
My son:
Notice how 'solid' Prince's hair is, versus my son? Natural hair color isn't 'solid' (it's why woman get highlights....for that depth and tone that kids have)
I like MJ. I'm actually kinda upset about his passing - but that being said, I think he bleached his son's hair.