Some background: I am a black American, DH is a white European immigrant and BM is a white American. SD (5) is white, DS (1) has brown skin.
The topic of race and skin color has started coming up for SD, and it's been my policy to just answer honestly. Recently, SD started calling DS "boy", as in "come here boy" in an affectionate manner. I told her not to call him that and she asked why. I explained that in the past, some people with white skin like hers, called people with brown skin like DS, "boy" to be mean. This opened up a whole topic of what DS real skin color was (she explained that he was actually tan) and that she wasn't trying to be mean. She seemed to take my simple explanation well, but it got me thinking about my role in this whole thing as I imagine these topics will come up more and more.
Ideally, I'd think her parents would discuss these things with her, but DH lacks the cultural context to address many of these topics and so it falls to me when SD is at our house. Does anyone have any experience with this? Obviously, when something like the "boy" thing comes up, I have to address it. Should I just leave it at only addressing things if forced to?

Re: Blended-blended family & race issues ?
Additionally, I've found that SDs come to me with that sort of thing more than they do with DH or BM. They feel more comfortable because I'm not a "parent parent" if that makes sense. I don't think they run back to their parents and tell them what I said because they are uncomfortable asking them on some level in the first place.
If you don't feel comfortable then I would defer to DH to answer. If you fefeel comfortable answering then I don't see anything wrong with that.
What does that have to do with the fact the her SD called her DS "boy"?
Kids that age don't know that calling a male child "boy" could possible be offensive. @HoolyGo I think you do need to discuss with your DH the way to handle these teaching opportunities: what to say, who will talk to her (you/him/together). Make sure he realizes that it is important to you and that you and he are on the same page so that if BM ever has questions/concerns your DH can handle it
And you are flat out WRONG that all BMs are crazy and haters. Or are you speaking from your own personal experience?
Hoolygo - I think you handled this appropriately and it sounds like it was received well. I say bring it up to your H but I do not think you need to address it with BM. All children of all races in all types of families are going to go thru all these kinds of conversations. Just have them in a healthy way. If you see a trend toward derogatory comments or concerning prejudice behavior, then yes, DH should speak to BM and uou all can have a healthy conversation around whats inappropriate and PC.
Not sure where you got the impression that I though SD was doing something intentionally wrong. I am well aware that it is completely harmless. I am also well aware that no matter how harmless it may be, a little white girl telling a little black boy to "come here boy" is not going to go over well in a group of black people. And therefore, it needs to be addressed (if only to save SD from getting into an ugly confrontation with someone outside our family.)
My question was really about dealing with these things in general as they come up (and they have been coming up in our household more and more lately - I am happy to provide other examples of needed). I imagine that it's similar to wondering how step-parents have dealt with other topics that could be controversial like sex, religion, etc.