I would argue that it hasn't evolved in the countries where the art originated but that the idea of the black belt was taken and changed into 'martial arts master' by returning US servicemen who brought the art back here. That doesn't mean that the idea has evolved; simply that it was never properly communicated in the first place.
Right, those servicemen who went overseas to study martial arts generally received their 1st dan in about one year of training. They enjoyed martial arts training and so when they left, they started their own schools, as 1st Dan "black belts". But their problem was how to promote their students. They did not have sufficient rank to promote to dan levels, so they began to lengthen the time required for promotion. If and when their students questioned their own one year promotions, fantastic stories of 8-10 hour training, seven days per week started coming out. Hence, the urban legend that black belt that takes 5 or more years to earn, when in fact, it took one year of ordinary training, and still does.