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 HEAVILYheavily 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.