Hybrid = having one recessive and one dominant allele
This means that all of the flowers must have the genotype Bb.
Knowing this, let's make a Punnet Square.
[tex]\begin{tabular}{ l c } & B \ \ \ \ b\\ B & \boxed{BB}\boxed{Bb} \\ b & \boxed{Bb\ }\boxed{\ bb} \\ \end{tabular}[/tex]
As you can see, 3 of these are going to have the dominant trait and 1 is going to have the recessive trait. This corresponds to our 3:1 in our question, meaning that the black seeds must be dominant and the brown ones recessive.
Because they are dominant, the genotype for black seeds could be BB or Bb.
