What is "the moon runs down in a purple stream" referring to?

Regarding this verse of the traditional folk song “Didn’t My Lord Deliver Daniel”: > The moon runs down in a purple stream And the sun refused to shine And every star did disappear Yes, freedom shall be mine I realize that the second and third line allude to the Biblical passage about the sun disappearing at the moment of Christ's death. But what about the first line? What is this an allusion to?