Origins
Few English words conceal as dramatic an etymological secret as 'bless.' Behind the word that English speakers associate with divine favor, grace, and goodness lies a much older and darker meaning — the ritual marking of objects and people with sacrificial blood.
The word descends from Old English 'blēdsian' or 'blētsian,' which meant 'to consecrate' or 'to make holy.' The verb is derived from 'blōd' (blood), with the suffix '-sian' indicating a process or action. The original meaning, supported by cognate evidence from other Germanic languages, was 'to consecrate by sprinkling with blood' or 'to mark with blood.' This referred to the pagan Germanic practice of 'blōtan' (to sacrifice, to worship with blood offerings), in which the blood of sacrificed animals was sprinkled on altars, on cult images, and on the participants themselves as a way of sanctifying them and securing the favor of the gods.
When Christian missionaries began converting the Anglo-Saxons in the sixth and seventh centuries, they faced the challenge of translating Christian concepts into a language whose religious vocabulary was thoroughly pagan. For the Latin 'benedicere' (to speak well of, to bless — from 'bene,' well, + 'dicere,' to speak), they chose the existing word 'blēdsian.' This was not random: the missionaries recognized that the pagan concept of consecration through ritual was close enough to the Christian concept of divine blessing that the word could be repurposed. Over several centuries, as Christianity displaced paganism, the blood-sacrifice meaning faded entirely, replaced by the Christian sense of invoking God's favor.
Middle English
This semantic substitution was so successful that by the Middle English period, no one associated 'bless' with blood. The phonological evolution of the word aided this forgetting: 'blēdsian' shortened to 'blessen' and then to 'bless,' making the connection to 'blood' invisible to ordinary speakers. Modern English 'bless' and 'blood' look and sound nothing alike, yet they are etymologically the same word.
The adjective 'blessed' developed a remarkable double pronunciation that survives in modern English. As a straightforward past participle ('the priest blessed the water'), it is one syllable: /blɛst/. As an adjective meaning 'holy' or 'fortunate' ('the Blessed Virgin,' 'blessed are the meek'), it is traditionally two syllables: /ˈblɛsɪd/. This distinction, though eroding in casual speech, preserves a medieval differentiation between verbal and adjectival uses.
The noun 'bliss' — meaning supreme happiness — is often assumed to be related to 'bless,' but the connection is uncertain. Old English 'bliss' (earlier 'bliþs') derives from 'blīþe' (happy, cheerful — source of the archaic 'blithe'), not from 'blōd.' However, folk etymology and centuries of association have made the conceptual link between blessing and bliss feel natural and perhaps inevitable.