[SwellJoe]

Race relations are complicated in the US (and in most of the world). A term that wouldn't seem problematic can become problematic because of its historical context. That's the case with many racial epithets in English...words that seemingly have no negative connotation can take on negative connotation when used for a period of time to insult or separate people based on their race, religion, origin, gender, sexual or gender identity, etc.

No one blames you for not knowing; all you're responsible for is how you respond and correcting future usage. The appropriate response is something along the lines of, "I didn't know. I will say X in the future. Thanks for the heads up." And, if you haven't been told what a better option is, you can generally ask without causing upset.