Muffy has an orange tail but since her mom has a blue tail, her genotype is Bb. If Muffy's mate has a blue tail (bb), her offspring can be a 50% chance orange tail and a 50% chance blue tail.
Punnett's square visualization:
Why 50% chance? Based on Punnett's square above, one square of the offspring represents a quarter or 1 out of four. Since there are two out of four turned out to have a genotype Bb which corresponds to having an orange tail, it is a 50% chance. The same goes with the blue tail phenotype with the genotype bb, it has two out of four possibilities or a 50% chance.