Horses that look brown are either smokey blacks or dark bays. Just like with other colors, there are different shades of bay, and sometimes it's so dark it's hard to tell. Smokey black(cream gene on black) looks brown.
Your filly's dam is definitely a bay pinto(black with agouti). So if she's bay, she got it from the dam. Aristocrat is black, which can't hide agouti, meaning he doesn't carry it. It is possible that Aristocrat carried cream, though I've never seen any examples of his offspring with cream, but I believe his sire was a buckskin, so he could have got the cream from him.
Even though your filly and Aristocrat both look black, they can carry cream, an example of this is Little Kings Black Velvet, who carries cream. But if you look at his pics, he looks coal black.
It is also possible that your filly is just growing a dull black coat. I have seen her in the show ring and she's a very pretty black
so it might just be that her long coat looks dull.
I would test for cream and agouti if you want to know for sure. If she comes up negative for both, shes just black. Sometimes you can tell a black horse carrying cream because they have the cream colored fuzz in their ears, but not always.
Either way, she's pretty.