What does Exodus 4:9 mean?
For those who will not respond (Exodus 4:8) to Moses' other two miracles (Exodus 4:1–7), God provides a third. Unlike the others, it does not seem that the Lord asks Moses to perform it beforehand. This might suggest that Moses is not one of those who needs a third proof; if the Lord worked miracles twice, He could work a third.This sign involves turning water from the Nile—the river sacred to the Egyptians—into blood when poured on the ground. Whether this is a literal transformation into body fluids, or merely a drastic change in color, the message is the same. This would prove the Lord's power over nature. That included the Nile, which was vital to everything in Egypt.
The Bible does not specifically record Moses performing this third sign. He may have done it for the elders of Israel (Exodus 4:30). However, it foreshadows the first plague (Exodus 3:20) where God transforms the Nile itself into blood (Exodus 7:19). For a week (Exodus 7:25) there was stench and death (Exodus 7:21).