Most likely you simply didn't dry it quickly enough. If you let it soak or sit wet it will darken. I suggest a hot water rinse. The hot water heats the brass a bit so the water evaporates more quickly. Roll the brass around in a terry bath towel. The higher humidity you have, the more effort is needed to dry it. It's bone dry where I live so hot water will flash off the brass pretty quickly. If it's more humid where you are, you can use an air compressor or a shop vacuum on blower mode to blow it dry. You could use a hair dryer or a heat gun. You could put it in a pan and stick it in the oven at a low temperature setting. You could lay it out on a mesh screen and leave it in the sun.
Brass has a lot of copper and it will tarnish over time. To prevent tarnish in longer-term storage, keep it in a sealed container with little air or oxygen. I find a ziplock bag with the air squished out of it will keep it tarnish-free for at least a few years and that's all I care about. If you wanted to preserve it for decades you could look into vacuum bagging or nitrogen-flushing, desiccants or activated carbon and so on. I shoot it and clean it again before I have to worry about all that.
I do recommend nickel-plated brass for carry ammo because it generally goes exposed for months if not longer.