Answer
Water absorbs red light. Deep underwater, any sunlight that has made it all the way there is cyan (red's complementary color), because the red has been absorbed on the way. Red blood absorbs cyan and reflects red, so deep underwater, photos taken with natural light make blood appear to be black.
If a camera flash is used, there is plenty of red light available to reflect, so the blood looks red again.
Work Step by Step
This is discussed on pages 514-515.
