Why are some dried (dehydrated) lime fruits black and others tan brown?

Question

Whole dried limes (Limoo, Omani) come in either black or a tan brown colour. Black has a stronger flavour but what the reason for the difference? Some say that black coloured dried limes are smoked and dried, others say that it is because black are dried for longer. I have also read that black-coloured limes have been soaked differently before drying. Example image of the different types here: http://www.azeliaskitchen.net/lentil-dried-lime-soup-shorbat-adas/