With regard to the black pellet carrier: To quote from, 'Umarex Replica Airguns:1996-2014', by Steve MacGregor, "Umarex explained that the tolerances between the pellet carrier, barrel and firing valve were so critical that it simply was not possible to apply a nickel coating to the pellet carrier and maintain an effective seal."
Also, from the same publication, "A small number of 6" 686 versions were produced in a "steel" finish in 2008/2009. This is a lighter, shinier version of the standard nickel finish."
This is not to be confused with the limited quantity steel PPC 1500 version that was produced in 2006.