Chocolate brown?
From the slipping description, I figured that the clutches in the transmission had gone bad. Since there are several clutches, sometimes shifting manually to other gears can limp you back home. Itxs been a while since the auto transmission was explained to me, so the details of that statement may be wrong, but the broad strokes should be good.
Does the fluid smell burnt? Since there is no combustion in the engine, I'm not sure how it would get black. I've seen brown transmission fluid.
If the engine was leaking oil, it may be low on the stick.