Even if my eyes are open, very often I'll not register anything visually until I finish the song - then its always a weird shock as my senses come back online. Occasionally I'll see very darkly, but its almost like 2d - mostly I'm not aware of anything but the act of singing.
I am not whether its chicken or egg, but when I'm singing less completely then I hear and see more of the outside world (or vice versa).