I've noticed that when showing blood in a number of older movies, the fake blood is always bright red. Why was this?
I know special effects have obviously improved over the years but it doesn't seem like blood would be that hard to make a color that looks real. So it seems like there must be some other reason.
Blood in horror movies has a rich history. Black and white movies had no problem with it and could pull it off easily. So much so that it is said that Alfred Hitchcock shot Psycho in black and white because he wanted the blood in shower to look "realistic" since he couldn't get the same effect with fake blood.
"What was once alarmingly "realistic" now looks either stylized (if it's a good movie) or fakey (if it's not so good)."
That quote best describes how I feel about blood in old movies. 70's movies as OP points out didn't look 'realistic' because there has been a general shift in our perception. As always, 70's doesn't lack cheesy movies and so blood may seem fake to us. Tarantino and Coen now generally use darker, grittier shade and this is now even an industry phenomenon.
If I had to pin it down to one reason it would be because of trend change. It is interesting to know that lens flares were despised by filmmakers who used expensive equipment to counteract the effect because to them it was just a nuisance, but today lens flares are ubiquitous and are thought to add realism.
Note - As for how the shade 70's blood was achieved I can say only two words - corn syrup.
This post was heavily influenced by this article: Color of Blood
I recommend you check out the link for a deeper feel. Also the pictures perfectly encapsulate the visual styles.
In addition to the previous answers, in some instances movies have deliberately created less realistic effects due to Standards and Practices or other market requirements.
They turn down the gore, including blood effects, in order to meet film or TV classification requirements.
This is actually discussed really well in the 2002 movie Censor, where the characters discuss how certain acts of violence are deliberately made over the top so that they are unrealistic, and how movies sometimes include gore that bears little resemblance to real life viscera in order get scenes past the censors.
Different formulas.
https://uk.news.yahoo.com/real-story-behind-recipe-fake-000600549.html?guccounter=1
It was a difficult process to perfect the color which took us until the 80s and 90s when blood finally looked like the real thing. Unfortunately this trend has been reversing for quite a while now. If you look at the new movie Abigail the blood looks brownish. In general though it looks now pinkish in newer movies a horrible trend that takes me completely out of the movie because it looks so fake. And unlike old movies they can't use the excuse that it wasn't possible to achieve a realistic look. What contributes to that is the "Orange and Teal" color grading. Look at Transformers. Shia looks like an Orange. A lot of this color correction unfortunately doesn't just make a movie generally look fake but also alters the appearance of everything. It is just horrible. Read the article "Teal and Orange – Hollywood, Please Stop the Madness" if you want to know more about it.
https://shootandcut.wordpress.com/2013/01/01/teal-and-orange-hollywood-please-stop-the-madness/
