During the short-lived media celebrity of the recent "blood moon," I spent some Internet time bringing myself up to speed on the phenomenon—as I suspect many others did. My interest as a lexicographer was to investigate why this celestial event is called a blood moon; thinking in the literal way that I do, and knowing the color of blood, I was perplexed at the disconnect. Blood, of course, is red—deep, vivid, saturated red—and the moon was not. It achieved a kind of Marsy orange, but it was not red.
I found that like many other fossilized expressions, blood moon is inspired by the Bible. Three different verses in the Bible (Joel 2:31, Acts 2:20, Revelation 6:12) mention the moon becoming "as blood" or turning "into blood" and it was a short and intuitive step from there for English speakers to move the modifier to the preferred place before the noun. So props to Biblical authors for coining a metaphor that has persisted for centuries. They had a gift for colorful turns of phrase, and they recognized, as we still do today, that the mention of blood is a surefire way to get people's attention. The Bible is in fact a tome spattered with blood. The figurative and startling mentions of it are at least as frequent as the mundane ones. Consult your Bible if you want to learn about rivers of blood, blood sprinkled on altars, blood smeared on doorjambs, the blood of lambs, goats, swine, and bullocks, and of course bloodshed—lots of it. The first five books of the Old Testament have well over a hundred mentions of blood, many of them icky.
The endurance of blood in figurative expressions that do not seem like a perfect fit for it is probably due to the nature of blood itself: it's a substance vital to the life of all vertebrates, yet typically visible only in traumatic or unusual circumstances. And then of course there is the color—vivid red. Blood red. The true color of it is often startling and moviegoers in the early days of color film were deemed to be too sensitive for it: stage blood, also called theatrical blood, rarely had the color or look of the real thing. Here's a scene from the 1963 schlock horror film "Blood Feast," in which the stabbing victim's blood looks comfortingly more like red enamel paint than anything that would issue from living tissue.
The want of verisimilitude notwithstanding, many are startled and squeamish at the sight of blood, real or fake, and visual representations of it leave an impression. Likewise, blood is a word that can turn a conversation. When we hear the word, we advert to it. The Pantone system of color matching has over 100 shades that include ‘red' in the name but none of them is called blood red, perhaps for reasons of delicacy.
In modern English, the majority of uses of blood are literal insofar as they are surrounded by verbs that typically accompany other liquids (such as spill, flow, splatter), but there are also frequent uses where metaphoric meanings are central (boil, shed, taste, curdle). The idea of blood as being elemental and central to human life is evident from the earliest English, perhaps informed by already extant ideas about it, such as Hippocrates' classification of human types according to the influence on personality of four bodily fluids, including blood.
Flesh and blood, a term in use before the 12th century, originally denoted humanity and human nature and was extended later to stand for bodily form. With a possessive adjective, and starting in the 14th century, one's (own) flesh and blood came to mean one's relatives—or, as we still now say, blood relatives, those who share common, and usually fairly recent, ancestry. Interestingly, consanguinity is attested in English around the same time. It is derived from Latin, denotes descent from a common ancestor, and is made from roots that mean "together" and "blood". Do languages arrive at this notion independently, or do they borrow from each other? However it arose, the metaphor "blood is relation" is firmly fixed in English, as pointed out in sense 5 of blood in the OED (first attestations in the early 14th century):
Blood regarded as the inherited characteristic (later as the vehicle of hereditary characteristics) distinguishing members of a common family, nation, breed, etc., from other groups. Now chiefly with distinguishing adjective.
There is a modicum of biological truth in this—relation by descent can be determined via substances in the blood, and blood test may sometimes indicate a test for parentage. But our modern knowledge of DNA and heredity has done nothing to dilute the power of blood to denote membership of a family, ethnicity, breed, or race. Indeed, the prevalence of blood to characterize such relations and the influence they exert might lead one to think that reproduction of species was accomplished by blood transfusion rather than by the meeting of sperm and egg. The expression "blood will tell" (meaning that the characteristics of one's ancestors, especially the bad characteristics, will reappear in future generations) was already being referred to as a trite maxim in the 19th century—but it is still alive today.
In the United States, the terms colored blood and Negro blood caught on as topics of literature in the early 19th century, and show dramatic peaks corresponding to the Civil War years, World War II, and the Civil Rights era. There is also a marked increase of usage in the 19-teens, perhaps as a result of the legal codification of the one-drop rule, a term in which blood reaches its metonymic apogee. The idea behind the rule, codified into many state laws in the early 20th century, was that any person with a single sub-Saharan-African ancestor—in other words, one drop of "black blood"—was considered to be black.
Blood as the vehicle of heredity got an additional boost in the heyday of the eugenics movement, mainly the early part of the 20th century. Here are a few apt observations from Searchlights On Health: The Science Of Eugenics, a 1920 book by Prof. B. G. Jefferis, M.D., PH.D:
Do Not Marry Downward.— It is hard enough to advance in the quality of life without being loaded by clay heavier than your own. It will be sufficiently difficult to keep your children up to your best level without having to correct a bias in their blood.
If there is a scrofulous or consumptive taint in the blood, beware! Sickly children are no comfort to their parents, no real blessing.
Those who are born to become insane do not necessarily spring from insane parents, or from any ancestry having any apparent taint of lunacy in their blood, but they do receive from their progenitors certain impressions upon their mental and moral, as well as their physical beings, which impressions, like an iron mold, fix and shape their subsequent destinies.
It seems likely that blood has the same startling iconic value in all cultures. By way of example, here's a depiction of the Hindu goddess Chinnamasta, whose spurting blood nourishes her own severed head, as well as her attendants.
I wonder, though, if blood (the word and the concept) has such a busy metaphoric life in languages more distant from English. A brief survey suggests that the word is a monosyllable in most languages, suggesting its elemental nature. It will be interesting to hear from readers familiar with languages in other families to learn whether blood plays such a central and vivid metaphoric role as it does in English.