This may be - and it's obviously an outrage as nobody can deny that the Union raised a lot of coloured troops and that this troops fought well.Perhaps more to the point is the history of the 1st South Carolina (Colored) infantry regiment. It was the first officially recognized black unit of the Union Army. It predated the Emancipation Proclamation. It was made up of self-liberated individuals from the S. Carolina & Florida coast that were formed up on Hilton Head Island in 1862. Every once & awhile I have run into someone who blithely claims that there were no Union regiments from SC. Like your quote from Mr. Bell, the conscious or unwitting attempt to erase the contribution of self-liberating people in the fight for their own freedom is ongoing.
But we talked about a decisive breakdown of rice production because of the activities of Union troops (coloured or not) in Carolina - and it's that particular point which is just hard to assess - and even more to find proof for it.