As for any survey, the challenge is self-selection. Even using a randomized method, participants have to decide to participate. A lengthy, somewhat intrusive (one presumes) interview about health and one's life is dependent on characteristics and qualities of each individual participant. Easy to mount an argument that individuals who have experienced much discrimination would decline the opportunity to share personal information with a research team. The team potentially being viewed as authority and elite. On the other hand, PEW research suggests it is actually higher educated blacks are more likely to report discrimination and perceptions of racism. 

HabFan410 raises a legitimate question experience and perception. Unless the experience is blatant, one must ask whether perception rather than an actual event(s) is being reported. Then again, a high level of perceived discrimination still indicates a problem.