The study examines the professional development of in-service advanced music teachers as a result of their participation in a three-year collaborative online course. A socio-constructivist theoretical framework was used for designing and implementing the course. Participants were 24 teachers (22 teaching instrumental music and 2 vocal music) enrolled in blended collaborative learning activities. A mixed research method was adopted for assessing the impact of the course on teachers' attitudes and skills, including quantitative data (a close ended questionnaire about the quality of the online activities) and qualitative information (focus groups and interviews). Focus groups were conducted with teachers for collecting their satisfaction and perceived professional enhancement. School principals were interviewed to investigate perceived changes of teachers' behaviours in their music schools. The findings highlighted the impact of the course for improving music teachers' professional skills. Attending the course was an opportunity to discuss student-centred didactic principles, teaching methods, and practices in music education. Participants reported the development of a metacognitive attitude, a reflective teaching approach and a goal-oriented method to curriculum planning. The key factors that contributed to the effectiveness of the course as well as implications for designing professional learning activities for music teachers are also discussed.