This horse tested Aa from UC Davis. I would have SWORE she was black. I think I even used her as an example of a faded black.
Just goes to show you, how hard it can be to tell without a test. I agree that keeping her blanketed or out of the sun with keep her darker, black or brown. Some people also have luck feeding paprika or black oil sunflower seeds to keep them darker. I haven't tried either. Oh one more suggestion, mineral deficiencies can also cause a coat to fade especially copper and selenium. You may want to try a low salt mineral supplement formulated for your area.
Here her thread that talks about her testing.
Iris | Equine Color Genetics