This study investigates different instructional designs to promote students' collective cognitive responsibility for Knowledge Building in blended university courses. Using an iterative, design-based research methodology with reference to the conjecture mapping technique, the blended learning design of an undergraduate educational psychology course was refined over three years in three design iterations. The iterations differed substantially in the embodiment of the Concurrent Embedded and Transformative Assessment Knowledge Building principle that engaged students in knowledge assessments and strategy assessments of their community's work. The design of the knowledge assessment involved face-to-face small group and whole class discussions in all three iterations. In the first and second iterations, students also worked online by writing individual reflections and contributing to a community portfolio. The design of the strategy assessment changed in each iteration. In the first iteration, the students' strategy assessment took place in face-to-face discussions; in the second iteration, students contributed to an online community portfolio; and in the third iteration, the strategy assessment took place in an online community portfolio and face-to-face discussions before beginning the course and in the online community portfolio in the middle of the course. Collective cognitive responsibility was analyzed in terms of productive and informative participation, interdependence between participants, self-regulation skills. The results show that the second iteration's design was most effective for fostering the students' collective cognitive responsibility, showing an increase in productive participation and self-regulation skills in the first part of the course and also an increase in the interdependence of participants during the course. Some implications concerning the relationship between the implementation of the CETA principle and Knowledge Building are identified for future directions of inquiry and for blended learning environments design.