Yep, sooty brass. If it doesn't come off in a vibratory tumbler, I can guaranty it will come out clean as a new case if you had a wet tumbling setup. But it is nothing to worry yourself about.
Those are brass cases with powder residue near the case mouth, that's all. My brass cases look similar, some powders and combinations of powder and pressure will put more residue on the case than others. It's nothing to worry about.
I have some 38 Special and 357 Magnum nickel plated brass that has been reloaded so many times the nickel has worn off is some spots (but that's a lot or reloadings...).