The alloy frame was finished in the same color, sort of a shiny blackish finish all around, different from the matte greyish frame and barrel.
Environmental influences, wear and storage can discolor finishes, especially the relatively crude utility finishes like on these guns.
Finishes also do not photograph particularly well, especially in uneven artificial light like in your photos. Moving the light source a few inches can create different color hues. Frankly, your pictures give me no idea what you’re concerned about.
So if you don’t shine a flashlight, it isn’t noticable? That would lead me to conclude it’s most likely a less-than-perfectly-even application of the original finish. Remember that aluminum alloys chemically can’t be “blued” in the traditional sense.
PS: Ok, saw the photos you just added. That frontstrap does look a bit odd. Maybe it was worn and some clown tried some touch-up cold blue on it, although from what I’ve heard that doesn’t hold well on alloy. Have you tried rubbing?
I think that is just natural fading and thinning of the dye used to originally color the frame black. Anodizing is actually almost clear, the anodizing is then dyed black before being sealed.
Over a period of time, some dyes will fade due to thinning from wear and sometimes begin to change color from UV exposure depending on their composition. I have seen some 1950's-1960's Beretta alloy pistol frames that were once black turn a golden copper color.
Back in the 1970's my father had the black anodized aluminum mounting bracket for his fish finder on the boat turn a bronze color from the UV light from exposure to the sun after a couple of summers.