I do not know your language, but at least in Polish the translation makes some sense.
They have the word "czarny" which means "black". "czarny kot" is "black cat", "on jest czarny" is "he is black". The latter is purely descriptive.
There is also the word "czarnuch" which is the VERY offensive word for Blacks, best translated by "negro".
Now, the last name "Czarnuch" is a normal last name, without any connotations to the color black (except probably in its etymology) and does not sound weird/offensive.
The translation of this capitalized word would naturally yield "Negro".
Crnjak (tsr̩̂ɲak) has usages in Croatian language, one of them is "black" wine, the other is black joke.
There might be a reference somewhere where it is used as a description of a person, but probably not in Croatian.
I did notice Google Translate hallucinates words, some very amusing, when I translate from Any -> Croatian (this happens automatically when reading Google Maps place reviews). There has been quite a lot of words that naturally map to Croatian but there's no text (outside of blogspam) that uses it on the Internet.
