I think it may be more of a function that you choose people you know — if that’s the case somehow encourage people to broaden thier search or the classic “pipeline problem”. For that, I’ve got nothing.
Not on the board level, but I use to work for a company where out of 250 employees, I was the only Black guy. I don’t think it was due to racism, but they were based in an area of TN where the overwhelming majority of people were White. I would also assume that most Black professionals who were raised and went to school in small town TN would leave as soon as possible.
When the company created a satellite office in a more diverse city, they had a more diverse talent pool.