It's because you have "the/that" (itu) qualifying all that precedes it.
(Kucing itu) (berwarna hitam):
(The cat) (is black).
You see that "itu" (the/that) is simply qualifying "cat".
And in "Kucing hitam itu", it qualifies the whole "kucing hitam" as a block (kucing-hitam).
Sentence decomposition: "the (black cat), as a whole: the black-cat.
So, it's the way to understand when the to be verb is implied.
Kucing itu hitam: the/that cat...black.
Kucing hitam itu: the/that black-cat.
In the first one, you see that the "to be" is implied, as something is missing to make it correct English:
The/that cat... black - The/that cat IS black.
"Tu" and "ni" aren't in the dictionary. I think there are rather used orally? (Bahasa gaul?)
The proper spelling (bahasa Indonesia yang baik dan benar) is "itu" and "ini".
(At least, it's safer to use "ini" and "itu" rather than "tu" and "ni")
Itu is both the and that. It can be used for both.