Wow, someone who spelt Hamish's name correctly!
Actually, when you are hit or cut it doesn't bleed right away -- it takes a moment for the blood to well-up -- it starts and continues until the blood begins to clot.
I think that scene was done properly; the camera pans right on Hamish when he's hit with the rock, you see a "dent", and then the blood starts to appear.
Just my two cents.