Well at the moment we only have one, single, solitary Maran, a hen. We've tried several times to import the eggs and hatch them out, but only ever managed to successfully hatch one.
This is her (the one on the left!)
She's called Twinkle Toes in honour of her bent toes, which I suspect is due to a vitamin or mineral deficiency in the parent stock. We've had similar problems if our breeding stock don't have access to
enough fresh grass, so during the height of summer we generally
feed a vitamin supplement if we are trying to hatch eggs, which seems to sort the problem out. Twinkle Toes has quite a reasonable temper, but an example of one isn't much to go on. We used to have Marans when we were in Wales and found that they would generally start off ok, but with time every single cockerel (rooster) turned nasty, and as they were quite big, meaty birds and we had a small child at the time we decided they weren't the best choice as a family bird. We did try to select the gentlest males from a batch, eating them in turn as they turned nasty, but we never got a strain we were happy with. The females were always ok though.
A friend of mine used to keep Marans many years ago in France and she always said they were nasty, without exception. I always found the meat to be excellent. But I think that breeds can vary tremendously and each breeder is going to add their own 'stamp' the birds they breed, and unless a very close eye is kept on breeding for type then that type is going to vary generation by generation. And there's also a very good chance that a lot of cross-breeding has been happening with wilder types with darker meat and not-so-dark eggs, which I suspect is what has happened with your Mum's chooks.
I always used be very concerned with 'purity' in breeds, but over the years I've mellowed a bit on the purity and become more concerned with genetic diversity and suitability for purpose and am far less keen on the idea of never adding new blood to the breeding stock, so long as you keep selecting for the traits you want.