Why did he bless Hashem here?
Rashi: This is for the great wonder of fire descending from Shamayim via Shlomo [at Chanukas ha'Bayis].
Radak: It is for helping him to complete another Sefer of Tehilim, like I explained above (refer to 41:14:1:2).
Malbim: This king's success is via Divine Hashgachah Pratis, among the wonders that Hashem does for Bnei Yisrael.
What do we learn from "Oseh Nifla'os Levado"?
Nidah 31a: Even the one for whom the miracle is done, he does not recognize the miracle 1 .
Radak: He shows His wonders for Yisrael, and not for another nation - for He is "Elokei Yisrael." The nations' gods do not do wonders.
Malbim: Hashem does such wonders by Himself, and not via a natural Shali'ach.
Lev Eliyahu (Shemos p.84): This refers to hidden miracles, that only Hashem knows them.