Just saw the comment and I don't think "the both" is grammatically correct. "Both of the cats" might be okay.
why is oba used here?
It's the nominative masculine non-personal inflection.
both the cats are black - should be correct
'Both cats are black' is perfectly good and proper English. I am a native, well educated, well spoken English person.
"Both cats are black" is also the main answer, so if it was rejected for you, I'm afraid it must have been a bug :/
this seems right in English, please change it.
Are black cats associated with misfortune in Poland as well?