EDIT: No, it really didn't rain blood during the Civil War. But it might have kept the hippies away if it did... Happy April 1st.

I hope the mods will forgive this, because it doesn't directly answer your question, but it came to mind when I saw your post.

The Civil War does of course have some rather high per capita body counts, and the war dead of the Civil War exceeds all US casualties in all other US military actions combined. The single bloodiest days and battles ever fought by US forces happened during the Civil War, and some battles exceeded the death count of some later wars. Wikipedia has a very handy breakdown and analysis of this, using commonly accepted facts and data.

This leads up to the infamous Rain of Blood at The Battle of Lookout Mountain. While the casualties were fairly low there, a combination of concentrated dead, a developing storm front, and strong winds created a passing rain squall that showered General Braxton Bragg's men rain tainted with the blood of their dead and injured comrades. Bragg would later note in his diary that "The singular event was a grim portent of Biblical proportions" and that "...the men are quite understandably disconcerted by this bloody rain."

It's hard to say with any certainty but there are a lot of events from Asia in the middle ages which have large populations in contact with efficient armies and brutal wars for centuries.

The Muslim invasion of India, for example, may have killed off as many as 80 million people at a time when the word population (to say nothing of India's) was in the neighborhood of 275,000,000.

But of course that's comparing a multi century conflict with one lasting only a few years.