Seeing as Hakadosh-Baruch-Hu knows everything, why did He see fit to tell Moshe that He will see he blood?
Rashi: To inform Yisrael that He took note of the fact that they were performing His Mitzvos, and that was why He would pass over their houses and save their firstborn from the plague. 1
Why did Hashem see fit to add the seemingly superfluous phrase "ve'Lo Yih'yeh bachem Negef"?
Rashi #1: To teach us that a. even though the plague will not affect your firstborn, it will affect any Egyptian firstborn who happens to be in your house, and b. that it will not affect your firstborn wherever they are, even if they are in the house of an Egyptian.
Seforno: Besides the actual slaying of the firstborn, the plague incorporated all sorts of suffering 1 that affected the other Egyptians (who were not firstborn). It was that suffering that the blood on their doorposts 2 saved Yisrael, for otherwise, the Jews would have been plagued by those other plagues together with the Egyptians (as we find in Vayera in connection with Sedom, Bereishis 19:15).
Rashi (on Pasuk 22): See 12:22:3:1*.
'Evrah ve'Za'am, ve'Tzarah, Mishlachas Mal'achei Ra'im' (as recorded in the Hagadah [Seforno]).
As the Pasuk writes in Yechezkel, 16:6 "va'Omar Lach be'Damayich Chayi" (Seforno). According to Targum Yonasan, the Jewish People mixed the blood of the Pesach with the blood of the Bris Milah (which they performed that night, also refer to 2:6:2:1 and see Ba'al ha'Turim) and the two bloods saved them.