To be honest, I'd really like to know more about what he could and could not see, exactly. One report suggests "blind" while another suggests "a blur".
Now a blur would allow you to see the horizon, more or less. Or maybe just the direction where the sunlight is coming from. So you have a reasonable chance of keeping the aircraft right side up without outside assistance. But if the pilot was indeed 100% blind he would not even see the real or artificial horizon and had to be kept right side up by sense and outside coaching alone.
The fact that he attempted three landings before the RAF scrambled a plane (or at least before the RAF plane formed up with him) suggests that he had some vision left, at least good enough to keep the plane upright, accept steering directions (I hesitate to call them vectors because I don't think he could see a compass) and get reasonably close to a runway, on final approach.
Well done all around. And I think 18 years of experience must have helped!