The answer is as simple as it is depressing - I use the now out of production GW Flesh Wash and Red Ink. Two washes that I have 1/3 of a pot of each, and have painted blood with over the past few years, yes years. Luckily you don't use much of that stuff so hopefully I'll be able to paint with them a few more years before I run dry.
What I really liked about these old as hell Inks was their natural glossy finish, and you could get the best blood by mixing various amounts of Flesh Wash, Red Ink and water. Variations ranging from fresh blood splatter, soaked clothing, the yellow'ish/red discoloration of fabric once it starts soaking up blood etc.
So the answer on my part is simple, I use a mix of Flesh Wash (ink) and Red Ink.
There are however ways to get somewhat similar results. My personal preference is dark red blood, so you could start out with something like GW Scab Red or Vallejo Scarlett red, and mix it with brown, black or purple each in tiny amounts to make shifts in shade and color. If you try mixing shades on the same surface you will get a realistic look as some parts of your model are going to be more soaked thru with blood than other areas just recently sprayed by a cut jugular.
You could, to achieve the slippery, sticky and moist look of fresh blood mix your paints with some Gloss Varnish as it will be pretty hard to put the gloss varnish on perfectly after you've painted all the blood.
The problem with painting blood is - it's damn fun, and you can get carried away fast, going from a fleshwound to a "Carrie at the prom" look. Try to imagine how much blood your model would be sprayed with, from which sides/angles would the blood most likely come from. If you have weapons that need blood, try to imagine how the blood would run down the edge of a sword held upright.
I use two methods of applying blood, one is more direct control, and one is a "spray" of blood particles.
The first method requires two kinds of brushes. One with short hair, could be trimmed or just a worn out brush. Apply the blood using a "stabbing" motion on the surface, to create the irregular splatter. You can also use a thin detail brush in order to paint thin streams of blood, flesh cuts, small drops of blood in the exact location you want the blood to appear on.
The second method, the spray, takes a little practice on a crap model or any other surface to make you learn how to angle the spray and control it. You will need a toothbrush. You create your blood mix, it needs to be somewhat runny in order for this method to work. Dip the toothbrush so that the blood mix gets soaked up - then angle the brush appropriately and drag your thumb across it to create the spray of small particles on your target model. As I said, you need to try this method out on something else than you model at least once to get the hang of it. You can also limit the area affected by the spray by using a piece of paper or cardboard and simply use it as a shield next to your toothbrush while dragging your thumb across it.