Are you talking about McCain’s adopted daughter from Bangladesh? She may have been born illegitimately, but I’ve never heard anyone mention anything about her birth parents.
That was the rumor-mongering during the 2000 primaries, that McCain had fathered an illegitimate black child. I just did a quick google check to see if there were still articles about it. There are. It was a well-known smear campaign, and often attributed to Rove.
I'm not saying any of it is true or not but this blog post describes the accusations against the Bush campaign pretty well. Rove was accused of orchestrating it.