Additional Study Information: The purpose of this phase I study is to test the safety and tolerability of ruxolitinib at different dose levels in combination with decitabine in patients with accelerated or blast phase Myeloproliferative Neoplasm (MPN), which is a group of diseases of the bone marrow in which excess cells are produced. You may qualify to take part in this research study because you have a MPN blood disorder called myelofibrosis (MF), essential thrombocythemia (ET), or polycythemia vera (PV) that is accelerated or blast phase.The purpose of this phase II study is to test the effectiveness of ruxolitinib in combination with decitabine in patients with accelerated or blast phase Myeloproliferative Neoplasm (MPN), which is a group of diseases of the bone marrow in which excess cells are produced. A fixed dose of ruxolitinib that was determined to be safe in the phase I portion of this study will be used in combination with decitabine in this phase II portion of the study.