Hi flboxer, The answer is he might get a little darker, but it will turn more of a reddish color then a black or brindle. But he's black hairs will never go away, gives them some character
. They will fade as he grows but if you look close enough you will always see them. You might also notice that the white on his face will become smaller as he grows into his markings, so if it a the white on his faces covers most of it, it will narrow down to a thin line in the middle of his face, same goes with the rest of the markings on the legs and check. Oh and the white on his check and underside may get spots on them as will, like little freckles.
Here is a link to color changes in boxers, hope this helps (linking to other Boxer boards not permitted by rules), just click on Boxer Coat Color, Thanks Melissa