Black was a material used for civilian sack coats of the period, hence you would have it as common material, plus it does not match what the Yanks were wearing...

When was the letter written for the clothing request?

The link is of a Winslow Hommer painting of Confederate Prisoners at Gettysburg...note that the artist did a good job of making the Confederate uniforms as a dark grey/blue color, much like the imported British Army Cloth we are talking about. Look at how it contrasts with the blue uniforms of the yanks.

http://crh.choate.edu/english/salot/Prisoners%20at%20the%20Front.jpg

Kevin Dally