This paper explores how a formative assessment-based blended English-as-a-foreign-language (EFL) listening course affects students' perceptions of their listening performance and what difficulties they may encounter during the course. The study was carried out at the Hue University of Foreign Languages in Vietnam. Sixty students majoring in English took part in this study. A 5-point Likert-scale questionnaire and semi-structured interviews were used to collect the data. The results show that the students positively perceived the use of online formative assessment in blended language classrooms. The findings suggest that the online learning platform supported learners in experiencing rich online learning resources, actively engaging a flexible, personalised learning environment and effectively practicing their listening skills. Regarding difficulties, the lack of technical skills, and anxiety related to social communication strategies were perceived as common challenges that limit learner engagement in the peer-feedback activity and in the collaborative learning community.