A meme originating from the Plagues of Exodus:
Moses turned the Nile to blood, demonstrating the power of Jehovah (YHWH) to Pharoh
The meme also appears in Revelation 16: Two of the angels that come bearing the last seven judgements/plagues turn first the oceans and then the lakes and rivers to blood.
Secularly, the term has come to mean "a lot of death," especially in sensational news headlines:
River of blood flows in war-torn Congo Chicago Tribune, July 10, 2001
And in poetry:

And a river of blood flows
Through the courtyard of the cathedral
None left to mourn, only the softly-falling rain

Log in or registerto write something here or to contact authors.