1.
It is King David who is blessing Israel in words - "Blessed is the man"
2.
Considering the phrase - "Blessed is the man..." in light of Tanakh mentioning, that King David came and sat before G-d (in the Temple). No one is allowed to sit before G-d in the Temple, so what that phrase means? It means that he sits in prayer.
Also parallels are drawn between the achievements of Moses and King David. In his final words in Torah Moses blesses Israel, and here King David blessed Israel too.
3.
The Creator does not deliver improper speech, so David modeled himself after the Creator. Therefore instead of saying - "Cursed is the man who walks...", he said - "Blessed is the man who walks Not ...
4.
The idea of blessed man should be considered in the light of words (in Tanakh) that God is the Sun and the shield...and in multiple places it is described how Avraham was rewarded for being upright, and also it is mentioned that the one who walks upright is rewarded, and the wicked is punished...
5.
The way King David begins Sefer Tehillim ("Blessed...") is the way Moshe ends his blessings. Our rabbis explain that it is not a coincidence, instead it is a principal established by G-d Himself.
In the beginning G-d Himself used to bless His creation. He blessed Noah and his sons, He blessed Avraham. But He also conferred the Honor of blessing to Avraham, so from that point, if Avraham felt like blessing, He could bless too. Later this honor was passed to Yakov, Moses and eventually came to King David. Every time the next great profit started, where the previous one ended. King David followed this tradition, therefore He started His book with the word "Blessed...", because that was the way Moses ended His blessing to Israel.
6.
... "wisdom strengthens the wise"(Kohelet 7:19).
Ten authors of the Sefer Tehillim:
Adam, Melchizedek, Avraham, Moshe, David, Solomon, Asaph, and three sons of Korach.
David strengthened more than all other authors, so the Book of Tehillim bears his name.
Ten kinds of song: glory, melody, Psalm, song, praise, prayer, blessing, thanksgiving, Hallelujah, and exultation.
Hallelujah is the most excellent song of all, which in one word contains the name of God and the term of praise.
7.
If a man walks among the wicked. he will end up stand among them, and if he stands with them, he will end up sitting with them and if he sits, he will scoff with them too. One good deed leads to another good deed, so one wicked deed leads to another wicked deed.
If a man does not do evil things, does it mean that its enough to do neither good or evil. No, His delight is in the Torah of God.
Rabbi Shimon taught if the man sit and not commit iniquity, he is rewarded as if he performed a commandment, rabbi Zeira said that such a man is rewarded only if an occasion for a wicked deed came to hand and he abstained from it, for it is said - "Depart from evil and do good... (Tehillim 34:15)
8.
The word Blessed is written twenty two times in Tehillim, according to the twenty two letters of the alphabet. King David prayed to God to accept the words of His mouth, and for generations to treasure them, as if He is still alive.
9.
Adam's and Eve's eating from the tree is shown as an example of walking, standing and seating with a wicked serpent. The serpent is demonstrated as a scoffer.