This is just a first impression, and just like a black belt it's self it doesn't always mean anything... but when I see a Black Belt, I look to see how much WHITE is showing through. And if it is natural wear or faked. (Fake is easy to spot, and it's done more often than you would think do it.) I also watch to see if their movements match the belt.
Ultimately though, it is the individual that matters... In many schools of old, colored belts were more for the instructor to quickly know who was at what stage to know who can work with who - and what aspect of skills they should be developing, etc.. I see Black belts similarly except that it does make a useful tool as a goal for students to aim at acquiring a level of excellence.
I really like what Steve Golden often says about it; "A Black Belt doesn't mean anything - Unless, you don't have one."