The Phase I part of the study aims to evaluate the safety, efficacy, and tolerability of subcutaneous (SC) blinatumomab for treatment of Relapsed or Refractory B cell Precursor Acute Lymphoblastic Leukemia (R/R B-ALL), to determine the maximum tolerated dose (MTD), and recommended phase 2 dose(s) (RP2D) of SC administered blinatumomab. The Phase II part of the study will evaluate the safety, efficacy, and tolerability of SC blinatumomab for treatment of R/R B-ALL and Minimum Residual Disease Positive (MRD+) B-ALL in participants 12 years old and greater. It will also conduct a clinical pharmacokinetic (PK) evaluation of SC1 and SC2 blinatumomab formulations.