While extensive research exists on peer feedback and its effects on writing, there are few experimental studies that rigorously investigate the effects of guided dialogic peer feedback on students' argumentative writing performance. This study, adopting a mixed-methods approach, examined the influence of guided dialogic peer feedback within a blended learning setting on the writing performance of 63 university students studying English as a second language (L2). The participants from two intact classes were randomly assigned as an experimental group (31 students) and a control group (32 students). Over a period of 18 weeks, the experimental group engaged in guided dialogic peer feedback instruction, whereas the control group was given traditional teaching. Both groups were assessed through writing assignments on two different topics, administered as a pre-test and a post-test. Additionally, students from the experimental group filled out a questionnaire and some engaged in semi-structured interviews after the treatment. The study's findings, derived from a series of t-tests and ANCOVA, revealed that dialogic peer feedback significantly enhanced the students' writing performance. Furthermore, the questionnaire responses indicated a positive student perception towards this instructional approach, a sentiment echoed in the interview analyses.