"There is a clock on the black building."
Translation:A fekete épületen van egy óra.
It is (almost) a valid translation, but requires a bit a weirder context. You can see that when you form the respective question by replacing the word in front of the verb with a fitting question word:
- Hol van egy fekete óra az épületen? - Where is a black clock on the building?
Instead of noticing that the building has a clock on it, you're trying to say in which spot a clock-on-the-building is.
One more thing: in the original sentence, the building is black, not the clock. :)