The importance of argumentative writing has long been recognized. However, many foreign language learners struggle to make effective argumentation and use appropriate language in argumentative writing. In this study, we proposed a blended learning design to address students' problems with argumentation and language use in argumentative writing simultaneously. The design consisted of offline collaborative argumentation tasks and online collaborative writing tasks. Drawing on a focal group of seven students, we examined the online and offline experiences of the focal group by analyzing classroom discussions, different versions of online writing products, and interviews with the students. Our findings indicate that the offline collaborative argumentation tasks enabled students to have thorough discussions and make indepth arguments, and that the online collaborative writing tasks helped students deal with their language problems in writing and internalize the language for argumentative writing. Overall, students expressed positive attitudes toward the blended design. The findings provide implications for future research designing blended courses on argumentative writing and beyond.