The thing that makes this technique worth using is that it makes the blood look realistic by letting the colors beneath show through. Even though it seems negligible, even having pink flesh show through beneath red blood can give the blood a lot more depth and thereby make it look more like a liquid than simple red splotches.
Over other sorts of colors, it's a lot more noticable. Here it is on my Crypt Haunter Courtier. Especially on the bone club, it makes the blood look more like it's flowing and pooling (the trick is of course that it is!).
I'll demonstrate with another Crypt Haunter Courtier I'm working on. Here it is before applying any blood colors. I'll be applying the blood in a similar pattern to the other courtier, on its outstretched hand, and on the bone club.
Start with Evil Sunz Scarlet (or whatever blood red paint you like to use). Get some onto a palette of some kind (I use a 85mm round base as a palettte). We need the palette because we need to water the paint down very very heavily until it is about 50/50 water and paint. Then we can get some on the brush, and apply it heavily to the areas we want to be blood soaked. Do one area at a time. After you apply the red, quickly rinse the brush, dry it a little bit, and then at the edge of the red (where it meets the flesh in this case) brush the red back toward itself to leave a fairly smooth transition between blood and underlying colors. Alternatively, if you like, you can get some less watered down paint on your brush and pat it on (stippling I think they call it) to leave a kind of splatter pattern at the edges of the blood.
Let this dry. It is presumably going to be very wet. Once it is dry, generously apply Carroburg Crimson (dark red wash) to the bloody areas. In general we want the Carroburg Crimson area to be slightly larger than the Evil Sunz Scarlet area. This will darken the red substantially.
Let this dry. It is once again very wet. The last step is to apply a thin layer of 'Ardcoat (gloss varnish) to the bloody areas. The gloss should be applied to the entire Crimson washed area, so that it gets a nice wet look even though it is only barely reddened.
And there you go, you now have a fresh bloody mess on your model.