Oh, yes, they can have a black mane and tail. We had one here for a while that you would have sworn was a blue roan because his mane, tail, and legs were jet black and his face was significantly darker than his body.
I saw a gray one time that was jet black and the only way you could tell it was gray is because there was a bit of graying in it's tail. Now, that horse was very young (like 2 or 3).
I have noticed that some will maintain some level of dappling for quite some time, then there are ones like Dobe that just turned almost all white over 3 years.