Thank you so much everyone. It is really great to read all your comments.
I should have warned that sewing on black aida really needs full sunlight or very very good artificial lighting as it can be hard to see the holes. I used to wear a white apron so that I could see the white through the holes which made them show up better.
Having said which I did find one really big advantage to sewing in white on black. I was half way through the second pass on a country when I realised that on the previous pass I had done one stitch out of place.
I really did not want to undo all the work to get back to it so I put it down and thought about it for a bit. And then took a black permanent felt-tip marker and painted out the offending stitch.
Now even I can't find where it was.