Originally Posted by SkyDog75

I'm suspicious that the Bianchi decals on the bike may not be original, which is making it awfully hard to identify.

Looking at other bikes of this one's approximate vintage, there were typically Bianchi decals on the down tube and seat tube, and a model name on the top tube. I haven't found any with Bianchi logos on all three of the frame's main tubes without a model name. The Bianchi logo on the side of the fork blade isn't typical, either, nor is the lack of a head tube decal. Is there a tubing type, frame size, or any other decal on the seat tube?

Without a positive ID of the year and model (or whether it's a Bianchi), there's still a good chance you can find a replacement derailleur hanger. You (or your shop) may just have to make a visual match from available replacements. We might be able to help, but we'd need a picture of the broken hanger (with derailleur removed, preferably). A picture of the frame dropout where it attaches would be helpful, too.

You'll also want to check the derailleur and wheel for damage. And try to figure out why/how this happened in the first place so you can make sure it doesn't happen again.