Well, I don't recall any company offering a front gold bead sight for the Blackhawk. Due to then being welded on,, there isn't a big market.
However, I have seen where someone milled off the front sight, leaving the base, dovetailing the base, and inserting a correct height rifle gold bead.
Some time back Ross Seyfried wrote an article and mentioned using a Pilot gold marking pen (Staples, Office Max, etc) to make gold bars similar to the custom bars inserted in the front blade. I tried it a it works great for long range shooting and the ink lasts quite a while. When it starts to wear just touch it up, it's almost like paint.
You might want to try it to make a square at the top of your blade and see if that works for you, much cheaper too.
Midway sells this jig. Dremel off the front blade (but not the base), and cut the DT with the jig. File, and check fit frequently, as you don't want it loose. Marble's probably offers a bead that will work (though Rugers are pretty tall). I've done a 686(shorter), but not a Ruger.