A bias blind spot is when we don't think we're biased, even though we may be. It's like an blind spot in our understanding and awareness of our own judgement. For example, say you think all people from one country, or one race of people, are all the same in some way. That is a bias, but it's possible you wouldn't think you are biased, even though you are. It's like a blind spot because you can't see your own bias.