It's the deoxygenated blood which is very dark and where there are drops of blood, the darkest drops are from veins and distinguishes where injuries are in other areas of a body, i.e. arteries.
Put a blob of venous blood alongside a blob of arterial and you'd see the difference.
Those dark blobs on Sheila's arm are, as I say, from her nose ( venous blood ) the blood coming from her neck is arterial, which is a lighter ( oxygenated ) colour as shown on her neck injury. Comparing the two, you can tell.