[Middle English blessen, from Old English bltsian, to consecrate. See bhel-³ in Indo-European Roots.]

The verb bless comes from Old English bldsian, bldsian, bltsian, “to bless, wish happiness, consecrate.” Although the Old English verb has no cognates in any other Germanic language, it can be shown to derive from the Germanic noun *bldan, “blood.” Bldsian therefore literally means “to consecrate with blood, sprinkle with blood.” The Angles, Saxons, and Jutes, the early Germanic migrants to Britain, used bldsian for their pagan sacrifices. After they converted to Christianity, bldsian acquired new meanings as a result of its use in translations of the Latin Bible, but it kept its pagan Germanic senses as well.

Source: The American Heritage® Dictionary of the English Language, Fourth Edition
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