Suppose you cross a homozygous blue flower with a homozygous yellow flower. in the next generation, all of the flowers are blue. what does this outcome tell you about the allele for blue flowers

The allele for blue flowers is dominate. If the blue and yellow were both homozygous (either FF or ff), and the blue is the only one that showed up (meaning it overpowered the yellow) shows that it is the dominate allele.