I have some stuff here that will turn aluminum blacker than the inside of a coffin on a moonless night I've done it on purpose on a couple of engines. The theory is .... The black gives up heat faster than the bare aluminum. Never had a temp gauge to test it though. It is a bit fragile until it "dries" after treating. I've soaked it in WD40 and then baked it to set the color and it seems to hold OK. The process is a cousin to bluing a gun.