Blood appears green at a certain depth underwater when illuminated by sunlight as you can clearly see in this video.
The reason for this effect is not that the blood itself is changing color, but rather because the sunlight is filtered by the water. Above the water, your blood looks red because human blood mostly reflects some green, but more red, giving blood an overall red appearance.
At a certain depth, the green component becomes dominant over the red component, and thus when this filtered sunlight reflects from your blood, the reflected light will look green.
Damn, that’s morbidly cool
Morbidly cool things are my specialty