It never occured to people because our knowledge of anatomy and it’s science is relatively knew.
Yes, they know blood, but from another, more spiritual perpective.
For example, Arjuna’s struggle were an inner one exactly because he had to fight his very blood. Now we use this a a figure of speech, but back than, it was really true. Only strenghtened by the teaching of Krishna did he manage to overcome the laws of the blood written and acting in his own body with more compelling strenght than we are able imagine today.
There comes a time for most when the path bifurcates: one side, the laws of blood, nature, and another one, that of the Spirit.
An interesting question would be, if the blood donated-given carries any particular astrality or not. There is an interesting law gave by Moses, what says: do not eat the flesh togheter with the blood what makes it alive …
and Pandara’s post seems to hint towards this direction, too.