In the religious sense, to bless is equivalent to magically grant [a] favour to someone - only those, usually gods, shamans, or holy men can do this.
1843 "A Christmas Carol" C. Dickens “God bless us every one!” said Tiny Tim, the last of all.
1855 C. Kingsley Westward Ho! I. ii. 68 ‘Matthew, Mark, Luke, and John, Bless the bed that I lie on.’
2016 Edoardo Albert "Oswiu: King of Kings" But my priest tells me Woden has blessed me. Ah, that we might only choose which blessings we receive from the gods.”
However, recently, my son asked his girlfriend's mother for her blessing to marry her daughter. She gave that blessing. In such cases, the blessing is a statement in favour of something, accompanied by the idea of making someone happy.
This use comes from a confusion of two verbs in Old English:
Bless: Etymology: Old English blóedsian , blédsian , blétsian
At a very early date the popular etymological consciousness began to associate this verb with bliss v. ‘benignity, blitheness, joy, happiness,’ which affected the use of both words (see especially senses 7, 8), and led to occasional Middle English spelling of the verb with i, y
Bliss v. Etymology: Old English blíðsian , blissian
**Bliss v. 2. transitive. To give joy or gladness to (originally with dative); to gladden, make happy. (In 16–17th centuries blended with bless.) Obsolete.
1634 C. Fitzgeffry Holy Raptvres ix, in Blessed Birth-day 50 To thee, who com'st from heauen to blisse the earth.
III. To declare to be supernaturally favoured; to pronounce or make happy.
- To pronounce words that confer (or are held to confer) supernatural favour and well-being.
This gives the supernaturally favoured effect
7.a. To confer well-being upon; ‘to make happy; to prosper, make successful’ (Johnson). Originally said of God; in later use also of men and things, but generally with an implication of their conferring instrumentally a divine blessing. (Here the association of bless with bliss becomes apparent.)
1549 Bk. Common Prayer (STC 16267) Matrimonie f. xvi* Look, O Lord, mercifully upon them from heaven, and bless them.
1578 Gude & Godlie Ballates (1868) 65 Bliss, blissit God, the good gift which thou has given to be our food.
This gives the "make happy" effect.
The end result is that anyone can bless anything.