Although interest in blended learning for students has grown within elementary and secondary school contexts, limited research exists on the implementation of blended professional development (PD) for teachers. The current mixed-method action research study examined teacher participation in blended learning PD in a large California district, offered in response to the challenges during the COVID-19 pandemic. Using a survey and interviews, the authors examined teacher self-efficacy and the reported influence of the PD on their practice. Findings suggest that the blended PD provided participants with the perceived self-efficacy needed to implement new instructional practices. In addition, collaboration among teachers was essential, teachers were able to learn through different pathways and at different rates, and ongoing support from a qualified PD instructor was necessary. This study sheds light on the implementation of blended PD for K-12 teachers, shares insights on practical considerations for planning and application, and suggests areas for future research.