Rat,
Part of your arguement relies on the assumption that English barrels were typically blued prior to browning becoming popular. I'm not sure this was the case. In fact, I don't recall seeing a blued barrel or any remains of blueing on an English gun from the time in question (mid 18th century). Yes, heavy cleaning is typicallly encountered on these guns, but if this was a typical procedure, I would expect to see some evidence left behind, somewhere.