This exploratory study aims to analyse the nature of peer feedback during a collaborative writing assignment, and to identify the possible effects of feedback on the revision of a text written by university students in a blended learning environment. Under analysis are two different graduate courses in academic writing, during which, over a period of a semester, the students (n = 85) were divided into 25 work groups to carry out a co-evaluation assignment with the support of a technology platform. The results obtained indicate that, when collaborative writing includes peer feedback, instead of unidirectional corrections from the teacher, the students respond more reflectively and constructively, they discuss the content they are working with, and, as a result, they make significant changes in their own writing.