I'm using a black oxide solution from Caswell Plating. You can play with dip times to get the color you want. Once the fastener is blackened, you can buff it with a shop rad and it will turn gray. I think it looks close to phosphate. I originally used Caswells sealer but it has to be applied thick in order to do its job. If you buff the fastener after applying the sealer, it will start rusting again. I've started applying RPM to all the fastners now.

I only mixed up half of the consentrate and it's still going strong. Half the consentrate will probably do an entire car. It was only $25.00