'd take somebody mighty used to the dark to make a competent witness . . .’

This shows how Mr Tate is trying to hint at how Boo Radley was there when Mr Ewell died.  He says that Boo Radley had spent a long time indoors.  Atticus misunderstood what he was saying and thought that it was an attempt to shift the blame from Jem to Boo Radley.