Blood. It's incredibly prevalent in film making. Whether it be for a death scene in a dark shroud of grim circumstances, or a valiant battle emblazoned with fortitude, blood will be an important part of the job. Blood can be incredibly complicated; it's messy, sticky, tends to stain (no matter what the label says), and varies in color, viscosity, etc. etc. etc. Not only are there a million choices when buying and making blood, but directors, art directors, production designers, wardrobe supervisors and makeup artists may all have a different concept of what the blood should look like, and that's to be expected. But, we all should know what blood actually looks like.
So what do you do if a character gets shot in the head? The heart? What if he's been attacked by a vampire? What if she gets cut? What if she gets cut deeply? Method, location, and circumstances will all factor in here, but should it be fancy HD yellow-toned blood or should it be classic, gory, gross, blue-black blood? Let's see...
In my experience, blood isn't nearly as deep blue-black as we all think it is. As someone remarkably clumsy, I've taken notice of what color my blood is, and although I really only see it from superficial scratches, I've discovered that non-veinal bloods are fairly yellow, and the veinal bloods tend to be a bit richer and cooler.
When you get to the nitty gritty, blood has iron in it. Iron rusts. Rust is red. When the arteries get blood from the heart it is full of oxygen, so it's a nice bright yellow red color. By the time blood is out of the arteries and into the veins, most of the oxygen has been taken up by important organs, leaving the blood deeper, with a bluish undertone. Arteries send blood out from the heart, veins bring it back, so the further you radiate out from the heart, bloods will adjust darker no matter whether arterial or veinal. It all kind of makes sense when you think about the last time you gave blood; They always ask you which arm has better veins, and the blood in the ampule (if you look) is pretty deep. All this knowledge is pretty impressive when you think about the fact that the Shining's Elevator had to bleed a lot of arterial blood...
Arterial=Bright Venous=Dark
There's a lot of great information out there about which blood is what, the colors they out to be and their coagulation time. But of course all that is highly subjective in film making. Sometimes, realism is king, and that's when it's a good time to pull out your anatomy and physiology text books, but most of the time, it's more about the look of the film, and the message meant to convey.
In any circumstance however, talk to your director before you show up on set about what kind of blood they want. Take photos, and send samples. Come to an agreement on what kind of blood and how much, so you don't have to bring 5 gallons of 10 different bloods to work to only use 1. And always bring shaving cream... Much easier to clean up!