Text of the Blessing over the Trees

Shalom uVracha. 

A. The blessing is over all the things that Hashem created which are not truly necessary for a person, but rather "for enjoyment." 

B. It also alludes to human beings, that they should be good. 

See the sources. 

Aruch HaShulchan, Orach Chaim, siman 226
One who goes out during the days of Nisan and sees trees that are putting forth blossoms says: Blessed are You, Hashem, our God, King of the universe, Who has not left His world lacking anything, and created in it good creatures and good trees to give pleasure through them to human beings — that is, he gives praise and thanks to Hashem Yitbarach, Who created for the sake of man even things that are not essential for human life, such as the fruit of the trees. Therefore we do not recite this blessing over grains and vegetables, for these are like necessities, unlike fruits; and one blesses at the time of blossoming, for then it is evident that they will produce fruit. And in our lands this is not in Nisan but in Iyar or the beginning of Sivan, and then we recite the blessing. And this blessing is recited only once a year, even if he sees other trees, for this blessing is a general blessing of thanksgiving for His kindness and goodness, may He be blessed. And regarding one who is careful with this blessing, about him it is said [Bereishit 27:27]: "ראה ריח בני כריח שדה אשר ברכו ד'" ("See, the smell of my son is like the smell of a field which Hashem has blessed") [brought in A.R.]: 

 And in the Haggadah of Ben Ish Chai (derashah for the month of Nisan) it is written that we mention "good creatures" in the blessing over the trees in order to teach that a person should learn moral lessons from the good, flourishing trees, and that "a person is a tree of the field," so that he too will be of a good character, to find favor and good understanding in the eyes of God and man. And he further wrote there that it also refers to the souls, for whom good advocates speak favorably.