i believe that the light will show the blood up as black and not red like the picture. However, you will have to have the light over the blood in order for it to work, you won't be able to stand straight up and shine the light down to get the blood to show. Like the CSI shows they use a chemical that reacts with the hemoglobin in the blood to shine when the light is on it. I think it could work for an emergency or if there is spotting of the blood.