I'll second the use of Wood Bleach (Oxalic Acid)
I put about a 1/2 teaspoon of the crystals in a quart jar of plain water.
Then heat the stirred mixture in the microwave for 30 to 40 seconds.
That warms it up just nice and it seems to work really well.
The directions on the mfg'rs packaging are to mix in hot water. But just warmed up seems to be fine.
I slosh it on with a piant brush,,even dunk pieces like the forend right into the jar if they'll fit. One end then the other.
You'll see the black marks and other discolorations go away right while your are doing this as well as the wood lighten up in color.
It won't bleach it to 'white'. Just lighten the orig color and overall even the color of the piece.
Flush with cool water and again use the brush gently.
The wet, warm stock wood can be easily bruised in this condition so be careful with it.
Shake it dry and then let it dry naturally.
It won't take very long to appear dry,,30 minutes or so. But let it dry over nite at least before doing anything to it further.
Resist the temptation to speed dry with a torch flame , stove burner or even leaving it in direct sunlight.
You don't want to chance burning edges of the inletting nor warping anything.
Here's the stuff I've been using since forever.
I bought it at HomeDepot and other like stores..https://savogran.com/pdfs/Wood_Bleach_PD.pdf
In the last year for some reason, I get strange looks when I asked for the stuff as it wasn't on the shelf in the normal spot in the Paint Dept.
They didn't know what I was even talking about.
So I did find a Deck cleaner that is Oxalic Acid. ,,and bought that instead. I think it's Thompson's brand.
It works just as well but is a liquid mix. I used it 1:1 w/water and it seemed OK.
There were 2 Deck Cleaners...The above and another that was for use in a power washer. This one contained detergent and a 'grit' to clean off the old finish....Not the one you want.
The first one clearly states..Active ingredient Oxalic Acid