Study of Elenestinib (BLU-263) in Advanced Systemic Mastocytosis (AdvSM) and and Other KIT Altered Hematologic Malignancies

The goal of this clinical trial is to evaluate elenestinib (BLU-263) in participants with Advanced Systemic Mastocytosis (AdvSM), SM with an associated hematologic neoplasm (SM-AHN), and other hematologic malignancies. The main questions it aims to answer are: * Determine Recommended Dose of elenestinib (BLU-263) monotherapy for participants with AdvSM * Safety and tolerability of elenestinib (BLU-263) monotherapy * Efficacy of elenestinib (BLU-263) monotherapy in participants with AdvSM * Determine Recommended Dose of elenestinib (BLU-263) in combination with azacitidine in participants with AdvSM * Safety and tolerability of elenestinib (BLU-263) in combination with azacitidine * Efficacy of elenestinib (BLU-263) in combination with azacitidine in participants with AdvSM The estimated study duration for each participant will be approximately 4 years: 2 years of treatment followed by 2 years of follow-up. Participants may be required to attend monthly visits for the first six months, followed by quarterly visits for the remainder of the study.