It’s funny
I think very often in fiction there is a romance between a dark-haired character and a light-haired character because it creates a sense of contrast and so on….. which just adds another one on the list of tropes appearing in sherlockhmm who might these two be
And then we have John and Mary (both blond) and Sherlock and Irene (both dark haired) I WONDER WHAT THAT COULD POSSIBLY MEAN