To be perfectly honest I'm sure there'd be some leve of unconcious bias towards people who look familiar or have simmilar characteristics to people I know. For me most people I know are white. Other factors like sex and age are a far bigger influence, but i'm sure race comes into it a little bit. I'm aware of prefering female doctors and there is one doctor I know who reminds me of my aunt so makes me feel a bit more comfortable. My aunt is white and and if the doctor wasn't she probably wouldn't remind me of my aunt so I guess in some way that means I am bias towards he because she is white, but only because of it's other implications.
Once you've dealt with a doctor a few times though that unconcious bias isn't so relevant cos you're judging them from your experiences with them. So many doctors I felt less comfortable with turned out to be ones I seek out for appointments now.
On a logical level I don't give a damn about what my doctor looks like as long as they know what they're doing and have good bedside manner. They could have a tail and horns for all I care. On a level of subconcious comfort though since I and most of the people I know are white there is probably some ammount of preference to white doctors.
Given a choice between a white male and a black female though I expect I would feel more comfortable with the female.