Racism is, more or less, an ideological perception of race, and so by virtue of thinking in such a way, you are then considered a racist. Much in the same way you are considered a Democrat for having similar ideals, or Christian for having similar beliefs. Exceptions do not eliminate the overarching concept at hand, and by having an African-American friend does not constitute a way of eliminating the possibility of the individual holding prejudiced thoughts, especially when the concept of "racism" is not limited to just one race.