I think the blind spot is not at b4, but it is really at d3.
It is not a "normal" square for a knight, and sometimes experience and pattern recognition can be a hindrance when there is something anomalous in the position.
For instance, we have all seen knights on f3 so many times that we automatically see ideas based on d4, e5, and g5, because there are a great number of patterns around those moves. We almost don't even consciously "discover" it- we automatically see it and factor it in to whatever we are thinking about at an assumptive level.
Not so with a knight on d3, where the pattern has shifted over a bit, and things aren't exactly where we expect them to be. It's almost as though there are shades of Fischer 960 chess appearing as shadows on the board.