Curricular interventions that embed blended learning technologies and pedagogies should support teachers' everyday practice. However, teachers' perspectives of what is happening "in the trenches" are often not systematically considered when evaluating curriculum and PD goals. Responsive online teacher professional development (ROPD) programs afford teachers and instructional designers with opportunities to collaborate on implementation. But in order for ROPD to be effective, curriculum coaches need to know what challenges teachers are facing. In this study, the researchers performed a qualitative inductive analysis to systematically identify and document challenges that teachers faced during the implementation of a blended learning curriculum. 56 middle school social studies teachers who implemented a new blended learning curriculum called GlobalEd 2 (GE2) completed 442 weekly online teacher logs, from which the researchers identified challenges that continued to persist for the teachers. The researchers identified 6 themes of challenges that persisted for teachers, divided into 18 sub-categories. These themes and sub-categories empirically provide a useful taxonomy and rich descriptions of the challenges that teachers commonly face in blended learning. In addition, the method developed in this study is a useful model for future ROPD programs to solicit feedback from teachers during an implementation of ROPD.