“10 Therefore David blessed the LORD in the presence of all the assembly. And David said: “Blessed are you, O LORD, the God of Israel our father, forever and ever….20 Then David said to all the assembly, “Bless the LORD your God.” And all the assembly blessed the LORD, the God of their fathers, and bowed their heads and paid homage to the LORD and to the king.” (emphasis mine)
Many are familiar with what are known as “the Beatitudes,” which are contained within Jesus’ “Sermon on the Mount,” recorded in Matthew chapter five. In that account, Jesus pronounces a list of “blessings” upon people who meet certain criteria (“the poor in spirit” have “the kingdom of heaven,” “those who mourn…shall be comforted” and so on). In the Beatitudes, what is this “blessed” state which is spoken of? It is a state of being “happy,” or “fortunate.” Louw-Nida states that it is “pertaining to being happy, with the implication of enjoying favorable circumstances.” Also, “First of all, Jesus is making an appeal to happiness. It is impossible to insist too strongly on the meaning of this makarios, repeated ten times (in Matt) and intensified by the present imperatives “Rejoice and be glad, for your reward is great in heaven.” This is much more than contentment; it is an interior joy that becomes external, elation translated into shouts, songs, acclamations. The explanation is that God will be the source of this beatitude.”
Therefore, we see that such “blessedness” is an inner state of happiness or contentment – what Jesus does in the Beatitudes is flip the worldly definitions and sources of “happiness” and “fortune” on their heads as he pronounces blessings upon those who seek things in antithesis to that which the world seeks.
How, though, does that apply to passages such as ours in question here – 1 Chron. 29:10, 20? Was David pronouncing “blessing” upon the Lord in the same way the Lord pronounced the blessings of Matthew 5? No, and here’s why.
There is the use of “bless” in one sense and then “bless” in another sense.
In the Old Testament the phrase “bless[ed} the Lord” appears 29 times in 25 verses. It can be used in the form of David proclamation of 1 Chron. 29:10: “Blessed are you, O Lord…” It is also used as an imperative – a command to be obeyed, such as its use in 1 Chron. 29:20: “Then David said to the assembly, ‘Bless the Lord your God.'”
Moving into the New Testament, we see uses such as James in 3:9, where he writes, “With it [the tongue] we bless our Lord and Father.” Does James mean that we are pronouncing our Lord and Father to be “happy” or “fortunate” due to our words? No. But why not?
Because there are two different words used in the New Testament and the Greek Old Testament (the Septuagint) for “bless.” One, makarios (in its various forms) is the word used in the Beatitudes. It is the one which means to pronounce happiness or good fortune (in a spiritual manner). Secondly, what James uses in 3:9 is a form of the word eulogeo. We should, upon seeing that word, recognize a word in our English. The word? Eulogy. The word James uses means, “to speak well of,” “to praise,” “to speak kindly of,” or “to extol.”
That is how man can bless God. That is how David and the people of God can bless the Lord in 1 Chronicles. Verse 20 actually gives us some help in defining our responsibility, because verse 20 states that the people “paid homage to the Lord.” “Blessing” the Lord means to “pay homage.” To give Him what He is due. Man, as a creature, should be speaking highly of God – he should be praising God. Because of his sin, though, man does not do so. Only upon repentance and belief will man be able to bless God. Three times in Psalms 103 and 104 we see the Psalmist declaring this: “Bless the Lord, O my soul.” our souls, as a matter of worship, are to indeed “Bless the Lord.” The psalmist in Psalm 134 issues a charge to the people of God to “Come, bless the Lord, all you servants of the Lord, who stand by night in the house of the Lord.” We see the New Testament, under the New Covenant, giving new meaning to “the house of the Lord” – said “house” being not a structure made with human hands nor bricks and mortar but a spiritual house, one comprised of the people of God. We, His people, should stand in that house – amongst His people – and bless the Lord, speaking highly of Him and praising Him.
Man can, but even more, must “bless” the Lord. That is the purpose for which we were created. Bless the Lord!!
 Louw, J. P., & Nida, E. A. (1996). Greek-English lexicon of the New Testament: based on semantic domains (electronic ed. of the 2nd edition., Vol. 1, p. 301). New York: United Bible Societies.
 Spicq, C., & Ernest, J. D. (1994). Theological lexicon of the New Testament (Vol. 2, pp. 437–438). Peabody, MA: Hendrickson Publishers.