Theatrical events, videos, and vampire LARPs (not to mention the occasional sick leave) can rely on the use of fake blood to impart realism to a scene. Here's a few recipes you might like to use if you ever find yourself in need of blood, and don't want to get it the hard way.
Realistic Looking Mint Blood
I've also been informed that Milk can be added (instead of or with the cornstarch) to keep the blood from being too transparent. White glue was also given as a suggestion, but if you go that route I wouldn't suggest using the mixture on or in your mouth.
Clear Blood for Wine Glasses
Real blood is foggy or opaque, but clear liquid looks better in a wine glass. Try this recipe:
Mix the Grendine and Corn Syrup through eachother. Add green food coloring one drop at a time, mixing thoroughly after each, until the 'pink' edge has been taken off the mixture. Pour into a wine glass, and swirl. The concotion looks very good under bright light, and moves with the viscosity of thick blood. If you plan to drink it, though, I recommend you cut it half-half with water.
I was promised the recipe years ago, but only came across it quite recently. It was worth the wait. The mixture may seem odd, but it tastes pretty good, looks surprisingly like real blood, splatters like real blood, dries like real blood, and had several people asking me if I was really okay after that staged fight....
Mix the cocoa powder thoroughly into the water before adding the other ingredients - it may help to use warm water. After adding the rest, blend the concoction well, and then wait for it to settle a bit. Either skim the bubbles & chocolate scum off the top with the edge of a kleenex, or pour the mixture into another container. The longer it sits, the more the cocoa tends to settle to the bottom, which oddly mimicks the effect of real blood seperating.
If you splatter this mixture onto cloth, it makes neat two-part marks which dry into pretty convincing bloodstains. If you let it run from a victim's mouth and then let it dry, the blood darkens and cakes to the skin in much the same way real blood does. I can also say from personal experience that any washcloth used to wipe down the 'bloody' face afterwards looks remarkably realistic, too.
Mix the Cherry dipping sauce with water, thoroughly enough to thin down the sauce into a gooey consistancy. Add food coloring. Stir again, and let the sauce sit, preferably in a fridge. When needed, take it out and spoon it onto areas where 'gore' effect blood is needed. The blood will drip in glops & globs, but doesn't puddle out like watery blood does.
Buckets o' Blood
Get a large pail to mix this all together. If you do not like the consistancy you can either thin it with more water, or thicken it with sugar or corn syrup. The exact amount of food coloring you require will depend on the brand you buy, so you may need to play around with the measurements. If you make it too dark, just add more water again. Adding some milk will reduce the translucent of the mixture (real blood isn't see-thru, but if you want clear blood, leave the milk out of the recipe). Don't add too much milk or the blood will look pink!
The final product should splash like water, but be slightly shinier, and not soak into cloth quite the same way water does, leaving more of it on the outside of clothes so they look suitably bloodied. NOTE: This will stain clothing, so don't get it on anything important.
Buckets o' Blood 2
This is great for the 'hands-on' type of blood, and (from what I hear) also works well in bath-tub sized proportions.
Directions are simple: Follow the instructions on the side of the jello package, but double or quadruple the amount of water needed, and don't add any sugar. Doubling the water gives you a very slimy, gloopy jello which doesn't look a lot like blood, but can be fun to get kids to stick their hands into at hallowe'en parties. If you use 5 gallons of water, you're going to have quite a thin runny blood, great for pouring over bloodied bodies in bathtubs or splashing on walls (especially if you can hose down the walls after - I wouldn't recommend this in your livingroom or parent's bedroom). You can play around with the recipe to get the desired consistancy - gelatin is reasonably cheap and available in almost any grocery store.