McClellan saddles starting with the M1859 were black leathers. They switched to brown/russet leather with the M1904.
Black leather was dyed that way. The change in 1904 was to natural leather which darkens to brown in strong sunlight and the use of things like neatsfoot oil. Just put a piece of natural tanned leather outdoors in the sun for a couple days and you'll see it darken a shade of more depending on how long it is exposed to the sun.Why, black and then to brown? Was it the cost or did someone finally figure out that the darker color would generate more heat and transfer it to the rider's *** and other extremities?