Blued steel back straps & trigger guards on the 1861 Navy are perfectly ok. In the 1851 Navies, blued steel grip frames are generally thought to be an indicator of a pistol made in England - or IOW a London Model. Back in the day, 1851 and 1861 Colt's Navies could be had with polished brass, silver plated brass, or blued iron grip frames. The Navy; for some reason, preferred blued iron grip frames while the Army preferred silver plated brass.
In the modern replica production era, brass is the most common grip frame material. There is really no rhyme or reason for what Pietta and Uberti use for grip frames. Currently, Uberti 4 screw, cut for shoulder stock frames are blued steel and 3 screw models not cut, are brass. 20+ years ago, they were the opposite. Piettas are blued steel now days; but have been brass (even some with square back trigger guard) in the past. ASMs were all over the place when they were made also. 2nd Gen and Sig Series Colts are 3 screw frames with silver plated brass.