I have a still life I would like to photograph that involves using fake blood. I am going to stamp a logo into the blood almost as if it was like a wax seal. I want to capture the red luminance of the blood and have been advised that I will need some sort of underlighting. I was initially thinking of spreading the blood out over the top of a lightbox (with ND covering the lightbox if neccessary) but was wondering how easy it would be to cut the rest of the lightbox out of the picture using photoshop (I don't want the white light as an element in the picture and was going to replace it with a black background). Ideally I would like to avoid having to change backgrounds in photoshop but am unsure about how I can use black when I need to underlight the blood. Any ideas?