I'm thinking the 'natural' one is simply uncleaned. I have quite a few Sikhotes in that size range, and after some gentle cleaning and an application of gun oil, they all have that dark appearance.
This group of Sikhotes appears how it arrived to me from the field collector, before cleaning or oiling.
A word or two about crusts on irons; what is commonly referred to as crust, is generally a layer of rust that is the result of long, slow degradation in the terrestrial environment. Sikhote-Alins, being a relatively recent fall (1947), frequently retain their original fusion crust. That is what appears dark, as fusion crusts in general do. Some Sikhotes show flight markings such as flow lines and rollover lips. Both of these features are present to some degree in the top image. The crust on these is typically thin, and rather delicate, you can often see where parts of it have flaked off revealing the thickness of the remaining portion. Any Sikhote with a dark crust like this should never be aggressively cleaned, ie. no wire brushing, and certainly no chemicals.
Sikhotes come in two varieties, the nicely crusted specimens you've pictured above, with genuine dark fusion crusts and an abundance of regmaglphs. These are generally the preferred individuals sought by collectors. The other kind is the 'shrapnel' pieces, which separated from the larger mass later in their flight through the atmosphere and had insufficent time for the surfaces to ablate, melt, flow, and form a crust. They can show a lot of surface detail, but it's related to stresses and fracturing.