The English words blessing and blood are closely related.
Old English blod came from P.Gmc *blodam, a word that in a still earlier form may have meant “to swell, gush, spurt,” or “that which bursts out.”
The English word blood has cognates in several other languages:
Old English bletsia, bledsian, bloedsian, meant “to consecrate, make holy, give thanks.” The P.Gmc form of the word was *blothisojan, “mark with blood.”
The word bless is unique to English.
Originally used for the act of sprinkling a pagan altar with blood, the word was adopted by Christian translators to render into English Latin benedicere and Greek eulogein which had been used to translate Hebrew brk, “to bend (the knee) in the act of worship.”
Towards the end of the OE period, bledsian took on the meaning “make happy” because of the word’s resemblance to OE bliðs, “bliss, merriment, happiness, grace, favor.” Bliss comes from a P.Gmc. word meaning “gentle, kind,” as does blithe.