Why Is My Blue Pitbull Turning Brown?

No that's not it. It may be genetics but that would show up right as the pup is born. His color won't change because of genetics. It has to do with sunlight and how well you maintain his coat. It's called coat rusting. Was he laying in the sun recently for a few days or weeks etc...?? The sunlight is naturally tinting his hair as the exposure to uv rays, his natural hair/skin oils and air pollution are tinting the color. It happened to my puppy at 8 weeks. He started to go all goldish-brown near his head and back. I kept him inside for a few weeks when it was really sunny or only put him in the shade. I got a special shampoo meant for blue/black coats or a gentle shampoo and even groomed him a bit and brushed him lightly everyday. Within 2-3 weeks, his coat was back to the greyish-blue I loved. I've since found that now that he's grown up, that his hair doesn't change as much as before in the sun. Its only when they are young. Feed him a good diet, brush his coat regularly and try and keep him out of direct sunlight for a while.

That's called coat "rusting". The brown is caused by old hairs that are soon ready to be shed. Sunlight and exposure to pollution in the air can cause a dog's coat to "rust", due to reactions with the dog's natural skin oils. Keeping the dog inside, or giving it a good brushing on a regular basis, will help remove the old hairs and stimulate new, "blue" hair to grow in. Some animal supply places even sell special shampoos designed for blue or black dogs that will eliminate coat "rusting".

Actually an American Pitbull Terrier that is true blue will have fawn or brown tint when in the light.  And actually "blue" is one of the most common colors out there now. There is not much unique about it.
Also "blue" is not a breed or a bloodline, it is a color of a APBT. You do not have a "blue that is fawn," you have a "fawn" American Pitbull Terrier.

It may be genetics, we have a blue pit also that is turning but he is more of a fawn color

Does it have a brown tint in his hair? I have a blue that has a fawn tint in her hair that you can see in the sun.