Call Ruger. You look at the BH manual and Old Army Manual you'll find the front sight base is the same part number as well as the blade. You can go to the Ruger website and download the manuals to verify yourself.
the front sight blades i have used are for a Ruger new model black-hawk. these work for the ss adjustable sight old army. part number mr03600 and mr03611(this is the higher one). i recommend getting both to test to see which you need. you can ether get them from midway for like 6 bux each or call Ruger and they will give them to you for free. you will also need a 1/16" drill bit to fit the blades and posibly a new role pin (part # KE-26). i used my old one.