Never underestimate the ability of humans to be WRONG.
Mentalists play with human psychology and the human mind like a fiddle.
Apparently, humans NEED continuity. They will excuse all kinds of changes to appearance -- even a race swap -- rather than accept the disturbing reality that something has changed that wasn't supposed to change (so quickly, a body double, etc.) We NEED things to be "normal" so badly, that we'd rather ignore a different skin color, than worry about "why the body double?"
This is just ONE example of how fragile our grasp on reality is, which can be exploited as Darren Brown does here: