Oh my God! You better never say "blackie" in the USA. I'll guarantee you that. That is a HIGHLY derogatory word and will easily be misunderstood unless you could find some very neutral context that very carefully made it clear you were not talking about people in the US of African-American ancestry. I can't even think of such a context myself, except for possibly the name of a pet, like Itzak mentioned. The word "black" by itself is just the color. So, that's fine. We used to call African-Americans "Blacks" in the 60s, 70s and 80s and that was the preferred expression at the time by people of African descent (it was sort of a neutral contrast between "Black, White, Brown" that sort of thing). Later, that got substituted for "African-American" (once again I might add because that expression was used for a very short time in the 60s). But "blackie" is very OLD and was always a very derogatory word for African-American babies, or small children. It's so old-fashioned now, I doubt anyone ever even uses it anymore. There are other more derogatory words out there still, but this is one you can toss from your vocabulary, except to know only if you're reading 19th Century books from the US like ""Tom Sawyer" or something. 

And here i thought it means to call a black person as cute and informal.  just like "kitty", "puppy", or "honey".LOL.

I was going to write that ¨blacky¨ is not a word, but it is in this dictionary:  http://www.dictionary.com/browse/blacky ; I urge you never to use it. 

Yeeeeah just forget that word lol.  Blacky would be problematic to use and fairly demeaning.

Blacky is the name of my dog cuz is cute and is a way to show my love to my dog ...