The answers to the clues below will have something  in common.

1. Metaphor for a rare occurrence
2. Time in Pablo Picasso's career
3. Pioneering George Gershwin piece
4. Democratic-voting unit
5. Police labor dispute strategy
6. Censor, delete, or edit
7. Thomas Gainsborough painting
8. Dreary workday, perhaps
9. Use vulgar language excessively
10. It might be rated R

Answers:

1. Blue moon
2. Blue Period
3. Rhapsody in Blue
4. Blue state
5. Blue flu
6. Blue-pencil
7. Blue Boy
8. Blue Monday 
9. Curse a blue streak
10. Blue movie