[Unclenick]

Yes. The Oxpho Blue used should work on both. It is phosphoric acid-based. Avoid hydrochloric acid-based blue at all costs. The chlorine weakens brass and steel.

I have used a number of chemical blackening agents on different projects and found that if you dilute them in distilled water 1:3, you reduce the aggressiveness of the acids on more reactive metals, like the zinc in brass. Dilute it and submerge the brass and use a retired toothbrush as an applicator and keep brushing all over until it is done.

I've found coloring brass cases is a great way to lose a larger number of them in the grass, but if you had some load it was critical to identify as unusual in some way, it will work.