[Thinlómien]

But the concept of fair hair is relative. I - a Finn - think I have relatively dark or at least "middle dark" hair, but if I asked anyone from say the Mediterranean region (or from Africa or Asia especially), they'd porbably call my hair dark blonde.

However, I think Brits and Finns have relatively similar concept of fair and dark hair, so I need not worry about that here. Anyway, I don't think all Rohirrim need to be necessarily blonde, but some of them might have red or light or middle brown hair. (So I could maybe be a dark-haired rohir... yay.) No folk or race in this world has only blonde hair (it would be quite unnatural) so I don't think the Rohirrim would have either - even if most of them were blondes. (Remember Gamling and Háma in the movies - they weren't blonde, but relatively fair and definitely Rohir-y enough if you ask me.)

As for specially mentioned dark-haired Rohirrim... what about Wulf (the one who Helm Hammerhand and Fréalaf Hildeson fought against) who had Dunlending blood in his veins?

And the Gondorians - I picture them at least relatively dark-haired. That would only make sense, because some of them have (even little) Númenórean blood in them and Númenóreans are dark-haired. Also the natives of Gondor - or at least some of them - are described as rather dark in various places: some parts of LotR and in Peoples of ME. Also, Gondor is in south, so if ME's climate resembles that of our world's, it would be only natural that most of the inhabitants would have brown or black hair unless they have moved there from further north.