Translation:These black houses are nobody's.
Good question! Really. I don't know if I can explain the difference though. I think "a fi al nimanui" goes deeper then "a NU fi al nimanui", at least for me.
When you say "Casele acestea nu sunt ale nimănui" it means they have no owner.
If you say "Casele acestea sunt ale nimănui" it means that in addition to not having an owner, they are abandoned and desolate, nobody cares about them, nobody wants them, nobody even wants to know about them.
"Casele acestea negre sunt ale nimănui"= " these black houses belong to nobody" To make you understand better I am going to replace " ale nimănui"( nobody's) with " abandonate"(abandoned) so you can see the meaning of " nu sunt ale nimănui".
" casele acestea negre NU sunt ale nimănui" becomes " casele acestea negre NU sunt abandonate" which means that those houses actually belong to somebody. So, " NU sunt ale nimanui" it means that something actually belongs to somebody.