This depends on your audience, of course, but bear in mind that for a lot of people, when they think of blood in large amounts, they're probably much more influenced by having seen it in movies; most people have never seen large puddles of human (or even animal) blood up close. For large amounts of blood in a movie, one of the most popular choices is still Kensington Gore, which is white corn syrup mixed with red (and a touch of blue) food coloring. In B&W movies, when it came up, they'd usually use anything dark; Alfred Hitchcock famously used chocolate syrup in the shower scene in Psycho. Some SFX artists use glycerin mixed with red creme makeup for close up, small amounts, and there are custom mixes you can buy from SFX and makeup houses.

My point here is that a lot of people will think it looks more realistic if it matches what they've seen in a movie than if it looks like actual blood.

That said, Arterial blood is going to be a bright red, whereas blood from a vein is going to be somewhat darker. You may want to swirl a little slightly darker red or reddish blue in the mix, to make it look like it came straight from the aorta and the vena cava together, or straight out of the heart. It also darkens fairly quickly as it sets up, and shrinks quite a bit as it dries. It's got a bunch of surface tension, so you'll want to the puddle edges to have a good curve to them, and it'll be just a touch thinner in the center, if it's free to spread out under its own weight.