Differentiating among great fielders is largely a matter of opinion as metrics are not reliable to make such differentiaion in a definitive manner, so there is no "proof". The metrics will separate the good from the bad and the bad from the average, but they don't reliably differentiate within those groups.
Blair is right at the top of the list of CFers that I have seen. He did it for a long time too. The Orioles of that era had 3 fielders that were among the best ever at their positions-- Brooks Robinson at 3B and Mark Belanger at SS were the other 2. Back to CFers, Mays was amazing and I only saw his last 5 years with the Giants. Curt Flood was a great fielder too. Freddie Lynn was spectacular- a human highlight reel. Darren Lewis was outstanding too. Fred, I realize that being an NL guy in the 1950's that you probably didn,t get a chance to see much of Dom DiMaggio. My dad told me that he was better than Joe D.
As for Duke Snider, anyone who thinks that he was just a one way player doesn't know what he is talking about. The debates in the 50's about Mantle , Mays and Snider were not just about hitting. While Mays was acknowledged to be the best fielder of the 3, it is my understanding that many considered Snider to be a better fielder than Mantle, and Mantle was pretty good at patrolling Yankee Stadium's expansive center field.
Bradley will never get the chance to be considered in the company of these greats unless he can hit enough to stay on the field for a number of years.