Poor Harley! Some things can cause a dark spot to appear on the beak, such as a bruise from an injury (flying into a window or other hard surface) and sometimes from nutrient deficiencies. Making sure Harley is on a balanced diet of seed and pellets appropriate for budgies (most pet stores will carry these) may help. Bruises will usually resolve on their own, however, If you think Harley may have been injured, or the dark spot spreads/peels/worsens, then having your vet examine her is best.