It might be that the bakers of the really dark stuff are using a caramel colouring, I have seen a couple of recipes that use it: https://artisanbreadinfive.com/2008/10/26/pumpernickel-bread-how-to-make-your-own-caramel-coloring/
I could imagine it might be popular with commercial/artisnal breadmakers as people think darker breads are healthier breads.