By faith he kept the Passover and the sprinkling of blood, so that the destroyer of the firstborn would not touch the firstborn of Israel.
God had made a promise. He had promised that if they put the blood on the door posts, then they would be saved from death. Were they trusting in the blood? No. The blood did not have any magical powers that would repel angels of death. This is not some formula that needs to be followed.
They were putting their faith in the promises of God. They were trusting in who God is. Our God keeps His word. His promises are true.