What version of Premiere Elements and on what computer operating system?
Does your plan include details of where the fake blood is coming from?
Is the blood on a separate graphic with transparent background? Or is the blood droplets and puddles already in the film as part of the black and white film?
If the latter, then what you seek is selective coloring in video. Is that the case?
The blood is already part of the shot, it is not edited in.
I still do not know your version of Premiere Elements and your computer operating system, but here is a plan that would probably work for Premiere Elements 11, 12/12.1, and 13/13.1 on Windows 7, 8, or 8.1 64 bit.
But you say that you are starting with a black and white image. If you could start with the color version of the image, I think we have a promising path to have the fake blood appear in color on a black and white image.
This would be done by applying the Color Pass effect to the color photo version. Color Pass Effect is found under fx Effects/Image Control/Color Pass.
Then you would edit the Color Pass effect under Applied Effects Tab/Appied Effects Palette/Color Pass Panel expanded. But I find that the most effective editing of that effect is done using the Setup whose icon appear to the right of the Color Pass Panel.
Have you used the Color Pass effect before?
I will offer how to suggestion for its use if interested. As for starting with the black and white, then I need to think about that some more.