Blended learning has grown rapidly in K-12 schools and is commonly seen as a potential vehicle to make learning more student centered by providing students with some level of control over their learning pace and path. As a result, blended learning is most likely to have a transformative effect when it is paired with constructivist learning strategies, such as guided inquiry, that emphasize student choice. In the research described in this paper, the authors examined one school district’s year-long professional development efforts to prepare social studies teachers and school librarians to design and facilitate blended learning units. They conducted 11 interviews with six participants and two focus groups with seven participants. Based on their analysis of the interview and focus group transcripts, they found that the professional development was effective at improving participants’ blended teaching knowledge, skills, and perceptions. Participants valued the facilitators’ feedback and modeling. They also found their interactions and collaborations with other participants to be valuable when attempting to apply their learning to their classrooms. Actually facilitating units with their own students resulted in the largest impact on their perceptions of blended learning.