I can't help but to notice the element 'dark' in it, but since it's of croation origin that might have no relevance.
When googling for that name, i cam across and individual who is seems to be well versed in Croation culture, and history, and his name is also Darko, Darko Zubfinic.
Here is his email, maybe he can email@example.com
It appears Darko is a last name.
Sometimes people on this site give refferences to this Kabalarian site, which give a 'philosophy' on what a name offers a person, that is to say, certain personality characteristics. It doesn't say what the name means, but here ya go:http://www.kabalarians.com/male/darko.htm
I found many Darko's while searching, and many with the last name, so your boy is not alone.