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. Why does fake blood in older movies look so fake?
Blood in horror movies has a rich history. Black-White movies had no problem with it and could pull it off easily. So much so that it is said that Alfred Hitchock shot Psycho in Black-White because he wanted the bloid 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 has lack of cheesy movies and so blood may seem fake to us. Tarantino and Coen now generally use darker, gritier shade 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 dispised 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 encapsulated the visual styles.