It's normal.
People don't think that copper plated bullets can copper up a rifle, but they're often astounded at the amount of copper fouling they get out of an AK-74 muzzle brake, and out of the gas block.
I found by accident that Shooter's Choice Extra Strength Copper Remover also does a great job of removing the black carbon from the muzzle brake and gas block.
I could never get the baked-on carbon out of my Bulgarian hard chrome lined brake no matter what.
I tried soaking it in Hoppe's, and with Slip 2000 Carbon Killer and it and the gas block were still black.
A short soak with the Shooter's Choice Copper Remover and they were down to the hard chrome lining.