Additional Study Information: This research study is a 3-part, randomized, double-blind study designed to learn more about the safety and effectiveness of an experimental drug called avapritinib (formerly called BLU-285), when given to individuals who have been diagnosed with Indolent Systemic Mastocytosis (ISM) or Smoldering Systemic Mastocytosis (SSM). Experimental means it is not yet known if the drug helps patients, and it is not available for sale. Randomized means that each patient has a random chance (like the flip of a coin) of being assigned to receive one of the treatments being tested during the study, and double-blind means that neither you nor the study doctor will know which one you are taking. Avapritinib is an experimental drug being developed by Blueprint Medicines Corporation to treat all forms of Systemic Mastocytosis (SM). It is an oral drug that is taken once a day continuously in 28-day cycles.