Transmission wont engage

Black fluid is usually burnt from clutches that slip. That is usually not the pump though, For that it's common for a seal or piston to bypass the pressure. There have been a few cracked apply pistons as well as failed seals in those units. Either one ends up as a rebuild to repair the damage.

You likely didn't notice slippage because of the way that transmission shifts. To make it smooth they program them the actually slip from one gear into the next. In the case of this transmission it was slipping in that area longer than it should and burned the clutches. That burns the fluid and makes it lose it's lubricity and that makes it slip even more until it finally fails to move. The heat in the fluid causes seals and valves to operate differently as well as age rapidly.