Is it safe to use clindamycin phosphate gel usp for moderate blackheads ?

Yes....but: If you have blackheads (also called open comedones) then the better treatment would be something like Retin-A (tretinoin). Benzoyl peroxide and salicylic acid products can also help with blackheads and are usually available over the counter. If you are in india i imagine a great deal of medicines are available at your local pharmacy.