There is no “below” that you mentioned but the gene combinations with bb would only be possible for blue eyes since big B (brown eyes) is dominant over b.
For example, if you were to do a cross and the father had Bb(brown eyes) and the mother had bb (blue eyes) the result would be
Bb,Bb,bb,bb. So there would be a 50% chance of blue eyes in the case :)
Hope this helped.