Ken,
I had the opportunity to watch Richard Sullivan charcoal blue a barrel from start to finish at the gunshop in Williamsburg a couple of years ago. I took a bunch of pictures and I did notice that when the barrel was finished there were some variations in the color. Once the barrel had cooled down and he wiped it with oil the color kind of evened out. I asked him if this was normal and he said yes, it is not as controlled as bluing on a modern firarm. The color was very pleasing and nothing like I have seen on any modern gun.
Michael