When the player has no effect, they are able to see the outline of shapes in the water fog / murkiness. 

However, if the player is given blindness while using commands and underwater, the player is able to see much easier and clearer than without the effect.

If the player is given the blindness outside of the water or swam out of the water and then swam back, the player will be blind at first, but then gradually will be able to see better