Greased Lightning is a multi-purpose cleaner that works very well on blood stains on bird mounts. It is the best that I have found. It is available at most the the big box stores. I get mine at Lowe's. I have also seen it in Wal-Mart. It's usually available in spray bottles or in gallon jugs. I usually get the gallon size and just pour it in a spray bottle.
I use it when I'm washing the bird, in the sink, along with Dawn dish-washing soap. Carefully inspect the bird looking for globs of dried blood, usually located in the down of the bird. Spray a liberal amount on the spot and kinda work it around and it will start to break down and disappear. It doesn't have any ill effects on the skin or feathers.
It also works fairly well on the black "gumbo" mud that is common on Snow Geese in our area.