Mark,
I don't know when the "color change" happened with Reynolds 531 but funny you should mention it because I was talking to Jimmy Thomson this evening and I mentioned all this old tubing I ran into lately. He asked me if there was any difference between the older Reynolds and the newer stuff. My reply was "I really don't know if there is a difference, but I prefer the older tubes, the stuff with the blackish finish. It makes me fell "good" and like I'm a kid again".
Obviously Mark has supplied the technical reason that older Reynolds tubing gives me that warm fuzzy feeling. I guess I'm too scientific for my own good! ;-o
Brian Baylis
La Mesa, CA