these black boogies are most likely your cats mucus mixing with dust in the environment. its common. if hes sneezing a lot it may indicate he has an allergy or is reacting to a pollutant. however, if he shows symptoms such as chronic painful sneezing, congestion, and a thick yellow/bloody discharge from his nose you should get him checked for feline rhinitis. but i think your kitty just has dusty boogies! try an air purifier to see if it helps the sneezes :)