Movie blood squibs are often something like a plastic baggie or bubble filled with either corn syrup or Methocel, thinned a bit, and colored with red and blue dyes to the correct color.
Methocel is used for a lot of slow-motion shots because it splatters so well (it's also used for slime, alien drool and the like), but corn syrup sticks to things more like realistic blood. They'll also just add in CG blood later, too.
Oh, yeah, and sea sponge chunks soaked in red make good organ bits, as do pieces of leather soaked in a dyed glycerin solution to make them slippery. They'll be thrown at actors and slip off, leaving red behind, for "Eww, I got hit with gore" moments.