It also depends in the light available when they would taken photographies. Black and white pics needed much more light to show a blond hair, with some red in it or more darker than a whitish Scandinavian blonde hair. Maria must have had darker hair when older, that's a fact. But maybe it was still blonde.
For example. When you sees a photo in black and white and a woman is wearing a green or red dress...you'll see it BLACK. It's the same with the "hair" thing. One of my great-grand aunties has a very light blonde hair, but sometimes, because of light, you'll see it plainly black...Colour photos are one of the better inventions of XX Century, no doubt!
RealAnastasia.