Academic writing, whether individual or cooperative, is an essential skill for today's graduates. However, motivating and helping students to learn to write effectively, either in cooperative or individual scenarios, poses many challenges, many of which can be overcome by technical means. The aim of this study is to investigate the effect of using cooperative and individual weblogs to enhance writing performance, based on blended instructional design. An equally weighted sequential quantitative-qualitative (two-stage) mixed research design was used for this study. In the quantitative dimension, the "pretest-posttest control group design" that was applied as an experimental study was used. In the qualitative dimension, the views of students in the experimental group were examined as a "case study." Twenty-one 5th grade students participated in the study, 12 in the experimental group, and 9 in the control group. While the experimental group worked on a cooperative weblog, the control group worked on an individual weblog. A writing performance exam was applied as pretest-posttest. Exam papers were scored with a writing performance evaluation scale. Results of the quantitative dimension indicated a statistically significant difference between the writing performance of students in the experimental and control groups in favor of the control group. When the sub-skills of students' writing performance was examined on the posttest scores of the "ideas & content dimension," a statistically significant difference was found in favor of the control group, but no significant difference was detected in the "sentence fluency, writing rules and organization" dimensions between the groups. Results of qualitative dimension showed that "feedback" has a positive effect on writing performance, unlike cooperative learning, which has an adverse effect.