Answer:
If blue flower colour is dominant to white then there will be two conditions for the dominant plant. It will either have homozygous dominant (BB) alleles or it will have heterozygous alleles (Bb). The phenotype will be blue flower colour.
As the white flower is recessive,it will have the alleles bb. The phenotype will be white flower colour.
If all the offsprings produced by the cross are blue, then it means that the dominant blue plant was homozygous for the trait. All the offsprings produced will have blue colour and a heterozygous genotype.