I don't think it's necessarily been blued.
I've seen many of this type of rifled musket that has, over time, developed a nice, rich brown/black patina that is the result of the decay of the natural oils that were often used on it and the stock.
Ok, I see that that has been dealt with. I shold have read the entire thread.
Yes, the guns were left in the white.
There are accounts of eyewitnesses during the civil war who could see bodies of troops approaching from miles away (certainly farther than the men could actually be seen with the naked eye) because the sun was gleeming off the gun barrels.