Be carefull hot blueing any double shotgun. the heat can soften the soulder conecting the barrels and alow them to seperate.
A finish I've been expermenting with is epoxy enamel paint. Where I live it rains over 200inches most years. Wind storms with over 100mph winds are frequent. Add to that the salt air that you have around the ocean and blue guns get rusty quickly. I've taken several rusty "boat guns" as we refer to them, and sanded and painted them with a epoxy paint. It gives a durable protective finish. As long as its in tact the gun won't rust and the black color looks good.
Try it you may like it. If not you can always strip the paint off. Regards, Byron