I used the Birchwood Casey Aluminum Black on a few of my raw AR lowers. I lightly sand blasted them first to rough up the finish, then I heated the lower until it was hot to the touch, put it in a gallon Ziploc bag & dumped a whole bottle of the Birchwood Casey Aluminum Black in on top of the lower ziped it closed & shook it to cover the entire lower fast. It got real hot from the chemical reaction but after I washed the bluing off & soaked it in gun oil & let it cool down it had a black scale covering it. I used a plastic spoon to remove the scale & it was good & black under the scale.
It was a little too much work so now I just use a black spray paint called VHT engine enamel. I just spray it on let it dry for 24 hrs the cure it by putting it in a 200° oven for 30 min. It is a tough chemical resistant finish that all the gun cleaners in the world won't take off.
https://www.vhtpaint.com/high-heat/vht-engine-enamel