Where there ever any blacks commissioned within the regular US army? Not until after WWII when the "colored" units were eliminated and the army integrated.
You are incorrect. There were black officers commissioned during the Civil War in the black regiments. The highest rank achieved by a black officer in the Civil War was by a graduate of Harvard University who was promoted to the rank of major.
You can argue if they were part of the United States Army or not. However, the first black officer to graduate from the United States Military Academy as West Point was Henry O Flipper, and he received a regular army commission because of that graduation in 1877 and was assigned to the 10th US Cavalry in the west.
Now the original question was: As the King's German Legion was part of the British Army, did the officers of those units hold the King's Commission?