God describes what he will do to Egypt if they don't listen as "the water which are in the river will be turned to blood":
Exo_4:9 And it shall come to pass, if they will not believe also these two signs, neither hearken unto thy voice, that thou shalt take of the water of the river, and pour it upon the dry land: and the water which thou takest out of the river shall become blood upon the dry land.
Exo_7:17 Thus saith the LORD, In this thou shalt know that I am the LORD: behold, I will smite with the rod that is in mine hand upon the waters which are in the river, and they shall be turned to blood.
Exo_7:20 And Moses and Aaron did so, as the LORD commanded; and he lifted up the rod, and smote the waters that were in the river, in the sight of Pharaoh, and in the sight of his servants; and all the waters that were in the river were turned to blood.
This article offers the idea that this is describing an algal bloom akin to the one now in Florida:
Giving credence to the idea is Joel's use of the same expression:
Act 2:20 The sun shall be turned into darkness, and the moon into blood, before that great and notable day of the Lord come:
I notice also that the Revelation uses the same figure:
Rev 8:6 And the seven angels which had the seven trumpets prepared themselves to sound. Rev 8:7 The first angel sounded, and there followed hail and fire mingled with blood, and they were cast upon the earth: and the third part of trees was burnt up, and all green grass was burnt up. Rev 8:8 And the second angel sounded, and as it were a great mountain burning with fire was cast into the sea: and the third part of the sea became blood; Rev 8:9 And the third part of the creatures which were in the sea, and had life, died; and the third part of the ships were destroyed.
Should we understand "blood" as referring to "the appearance of blood" in a Red Tide?
KJV unless otherwise noted.