Who is giving the blessing?
This is a typical Hebrew doxology, where the ones giving the blessing are implied (i.e., His people and creation) and "Blessed" follows the definition "revered, honored in worship, praised, extoled, exalted, magnified." So it is in Romans 9:5 of Christ. Paul uses the same form of doxology in two other places: (1) Romans 1:25, "and worshipped and served the creature more than the Creator, who is blessed for ever (ὅς ἐστιν εὐλογητὸς εἰς τοὺς αἰῶνας ἀμήν). Amen," and (2) 2 Corinthians 11:31, "The God and Father of our Lord Jesus Christ, which is blessed for evermore (ὁ ὢν εὐλογητὸς εἰς τοὺς αἰῶνας)." I've also noted this before.
Who is blessed
θεὸς εὐλογητὸς in this construction follows one of the simplest most basic uses of the Greek adjective, which is learned at quite an early stage. The two nominatives together like this means "God is blessed" since the adjective is in the predicate position (i.e., no article). The addition of is or be may be dropped depending on the actual construction, but the underlying meaning does not change. This is why I keep saying your interpretation requires an emendation of the underlying Greek. The text as Paul has written it--without inserting punctuation into the Greek to force a doxology to the Father--has "God" as a predicate nominative/apposition to Christ (i.e. Christ is God). Thus, together with "Christ" it means "Christ . . . who is . . . God is blessed."
Lastly, because the whole phrase ὁ ὢν ἐπὶ πάντων θεὸς is substantivized by the article that references ὁ Χριστὸς, the full construction becomes "Christ came, who is over all, God blessed forever." Thus Christ is "over all," is "God," and is "blessed forever." I can no longer add is or be in italics as above because the passage would be misread as a doxology to the Father.
