You could try Birchwood-Casey Aluminum Black. Chemical reaction, like bluing on steel. Darkness seems to depend on how hard the aluminum is. Sometimes nice and black, sometimes dark gray is best it will do.
Maybe someone will post that has tried it on an OM BH.
I have used the Birchwood-Casey Aluminum Black. It worked ok but usually takes two to three applications to get the bare spot really black. It also depends on how good you degrease and prep the parts. I clean them in acetone or denatured alcohol. I generally use it on my sight bodies just to keep the glare of the worn spots down.