If you really want a "blue jean", take a gander over at Hamilton Dry Goods (Mr. Hamilton is an NCOWS member). He sells a replica 1873 Levi blue jean that is period correct and more importantly . . . very comfortable.
As noted above, most 1870 contemporaries who wore blue jeans were either miners, or farmers.
I don't have a persona based in any time or reality. I'm just Pancho, "the guy who finds neat, cheap stuff on eBay and wears it to NCOWS matches". Soon, my main match guns will be 1890's revolvers, 1880's shotgun, & 1860's rifle.