This is NOT racism. It is nit even prejudice. While some people might be nervous when they see a black person, which is a coaches reaction from the media, it often does not mean that they have made a judgement on that person (yet) and their head is ruling over their fears and they are willing to engage with the black person. To be prejudiced is to pre judge someone; the white person in this case will not have formed a judgement her and cannot therefore be prejudiced. Also to be racist means to treat someone differently based on the colour of their skin. Just because you are nervous when someone approaches you, it does not automatically follow that you will treat the person differently when you have heard what they want, or why they have approached you.
It is a problem that people get nervous when approached by black people. The media teaches us that black people are in gangs or are violent. Some are, but I’m sure just as many white people are violent or in gangs and they get less press for it. That’s racist. But that’s the media we have. So some people might look a little nervous when approached by a black person. I saw it myself when I was a teenager in Guildford which is very white; and I am only mixed race. But people who are initially a bit nervous around you are nit racists. They are not lost causes. Engage honestly with them and see how many engage back. You can’t write someone off just because they are nervous.