Yes if they would have believed in the blood of the Lamb, which was the token of the promise of the Lord to the Israelites, the Egyptians would have been spared.
However this is just shoulda-woulda-coulda and sympathizing with wicked Egypt. They did not believe in the blood of the Lamb for there was not a house from Pharaoh to all of his slaves, all of the Egyptians, that did not have one dead says the scriptures.
Exodus 12:29-30
29 And it came to pass, that at midnight the Lord smote all the firstborn in the land of Egypt, from the firstborn of Pharaoh that sat on his throne unto the firstborn of the captive that was in the dungeon; and all the firstborn of cattle.
30 And Pharaoh rose up in the night, he, and all his servants, and all the Egyptians; and there was a great cry in Egypt; for there was not a house where there was not one dead.