Blood is complicated: it changes texture, color and thickness depending on where it's flowing from and how fast. Though you can make a convincing fake blood with water, food coloring, flour and corn syrup, getting the proportions right depends on a scientific understanding of the context in which your fake blood is appearing.
For example, if you want to simulate the blood of a geyser-flowing limb amputation, you want to go darker and less red than an old-fashioned blood spatter, and the drier the blood is, the more coagulated (and thus browny-black) it becomes.
Robbie Gonzalez's guide to the science of fake blood is a great resource for adding much-needed verisimilitude to your costume, party, or haunt.
Blood's creepy phase changes might be the hardest thing to reproduce in your blender at home, but thickening agents—like all-purpose flour and cornstarch—can help you dial in a more believable consistency. (Plus, they have the added benefit of making your homebrew blood less see-through and more opaque like the real thing.) If you're adding to the cup of corn syrup that you started with, try adding your thickening agent one teaspoon at a time.
Continue varying your proportions of water, syrup, dye, and flour until you've produced a batch of blood in just the style you want. Don't be afraid to experiment: Try a few different formulations. Use dark corn syrup, or chocolate syrup, instead of the clear stuff, for a darker base mixture. Just remember: Like the real thing, fake blood has a tendency to stain. So wear an apron and keep your blender's lid on—unless you want your kitchen to look like a crime scene.
Water, Flour, Syrup, Dye: Mastering the Elements of Fake Blood [Robbie Gonzalez/Wired]