I think the reason the blood looks purple is that it might have been a bit thin. Try using less water, or a thicker/more opaque recipe. Also, don't be afraid to keep layering on the blood until it really soaks into the spot.
It's nontraditional, but for this zombie outfit
I used Liquitex acrylic paint and gradually mixed in water until I had the right consistency. (It's also what I used on my skin and in my hair. It comes off skin pretty easily with soap and water but is a nightmare to get out of hair).
You might want to get some pants you don't mind getting dirty, too... if you're killing zombies, the blood is going to get everywhere, including your pants and skin. I also second the idea of looking a bit grittier overall.