Yes, they don't accept it here, but in some context, you can have "kucing hitam" translated as "a black cat", or "the black cat".
Anyway, if it was in a sentence, you couldn't have only "black cat" in English, you would need an article. So both are valid translation.
The black cat is beautiful = Kucing hitam itu cantik.
I have a black cat = Saya punya kucing hitam.
Thus, they should accept both.
I reported it, as "the black cat" is still not accepted.
But they do accept "a black cat". So it's probably a forgetting.