"God hath blessed thee for ever" in the Greek OT Ps. 45:2:
εὐλόγησέν (verb) σε (object) ὁ θεὸς (subject) εἰς τὸν αἰῶνα
Because God is the one performing the blessing, a verb is employed and an object in the accusative (σε) receives the action of the verb.
Deut 2:7
ὁ γὰρ κύριος ὁ θεὸς ἡμῶν εὐλόγησέν σε ἐν παντὶ ἔργῳ τῶν χειρῶν σου
For the Lord (subject) our God has blessed (verb) you (object) in all the works of your hands
Deut. 15:6
ὅτι κύριος ὁ θεός σου εὐλόγησέν σε
For the Lord your God (subject) has blessed (verb) you (object)
(see also Gen. 49:25, Deut. 12:7, 15:6, 14)
True Parallel to θεὸς εὐλογητός in Romans 9:5 - Psalm 67:19, 20 LXX
Note that the LXX verse numbering differs from English
κύριος ὁ θεὸς εὐλογητός εὐλογητὸς κύριος ἡμέραν καθ᾽ ἡμέραν κατευοδώσει ἡμῗν ὁ θεὸς τῶν σωτηρίων ἡμῶν διάψαλμα
"The Lord God is blessed, blessed be the Lord day by day, and the God of our salvation shall prosper us."
(Also, the construction εὐλογητὸς ὁ θεὸς, which occurs numerous times in the OT, does not differ in actual meaning, but in English we follow the word order for emphasis, "blessed be God.")
Other constructions ("Blessed by...")
καὶ νῦν σὺ εὐλογητὸς (adjective) ὑπὸ (preposition) κυρίου (genitive) (Gen. 26:29)
"You are now blessed by the Lord"
εὐλογημένοι (verb) ὑμεῗς τῷ κυρίῳ (dative) (1 Sam. 23:21; 2 Sam. 2:5; Ps. 113:23 LXX)
"Blessed are you by the Lord"
εὐλογήσουσιν (verb) γὰρ τὸν θεὸν (accusative) τὸν ἀληθινόν (Isaiah 65:16)
"Will be blessed by the God of Truth"
εὐλόγησεν (verb) ὁ θεὸς (subject) Ισαακ (object) (Gen. 25:11)
"God blessed Isaac"
ὁ θεὸς εὐλόγησέν με (Josh. 17:14)
"The Lord (subject) has blessed (verb) me (object)"