Use a piece of glass instead of the light box. Light it with lamps(S) from underneath. Use saw horses or chairs for support.
Drape the set up in a black cloth, with a hole in it that matches the blood spill.
Hold the cloth in place with "A" clamps, positioning the cloth to taste.

Hello Ronk,

Thank you very much for your advice. I understand the part about using glass and lighting from underneath. However, if I drape black cloth over the top of the glass and cut holes in it to let the light from underneath reach the blood I will have to be very precise with my blood pattern. This may make the blood (particulary if I want splatter) look staged and it will also mean that I cannot re-shoot different blood setups without re-cutting the cloth. Also, I would imagine the blood will seep into the cloth edges around the holes and this might be picked up on camera. It may also mean me having to apply more and more blood as the scene changes from it absorbing into the cloth regulary. Is there any way I could put the black cloth underneath the glass and move the underlights to the side of the work area slightly so they're out of shot, then I could shot through the glass all whilst avoiding glass reflections at the same time?! Sorry, its probably a tall order! Many thanks for your help.

Paul

True enough, I had not considered that aspect of the problem. One solution might be is to make the blood into a solid or semi solid. Using colored acrylic spills or a hardening agent on the liquid.
I have seen acrylic spills and drops for sale by a company called Tengove (spelling?).