In To Kill a Mockingbird, who is blamed for trespassing on the Radley's Place and what does it reveal about the setting of the novel?

It is Boo Radley who is usually blamed for any "small
crimes committed in Maycomb," but since Mr. Radley must have known that Boo was inside
the house (or at least on the porch), he blamed the trespassing on the next logical
choice: A Negro. It was not likely that any black man would be lurking at night on a
white man's property (since a shotgun blast was the probable result), but Mr. Radley's
guess as to the color of the culprit's skin would probably have been echoed by most of
the other townspeople. Miss Stephanie's crude attempt at humor was an attempt to make
light of the situation:

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"Shot in the air. Scared him pale, though. Says
if anybody sees a white nigger around, that's the
one."