The National Ski Areas Association has a guide to go by. There is a wide range of pitch angle for each classification(which I have forgotten). I believe that a black is from 35 to 45 degrees. The ratings only go up to Black Diamond. There is no mention of what a double black is. So if the slope has a pitch greater than 45...wow-we must have a double diamond!!! The slope and trail conditions should be determined over a majority of the entire trail.
There is leeway due to trail conditions (moguls, glades).
The big fudge factor is when one ski area is fighting the publicity war. Example- 1 -a ski area went from 102 trails to 134 over the summer without cutting down one tree!!! 2- a different area changed their black to a double black because the local competitor had cut a new trail.
You will find that most ski areas divide their classifications into roughly thirds.