Indiscernible things can't be seen or sensed clearly. A black cat is indiscernible in the dark, and the tiny amount of gray in your hair might also be indiscernible.
Flavors are indiscernible if you can't taste them, and objects are indiscernible if you can't see them. Your next door neighbor's piano playing is also indiscernible if he plays quietly, with the windows shut so you can't hear. The adjective indiscernible adds the "not" or "opposite of" prefix in- to discernible, from its Latin root discernere, "to set apart," or "to distinguish."