With the growing trend of Internet-based education, blended learning has become increasingly common in general education schools. Therefore, preparing pre-service Physics teachers to design blended lesson plans plays an important role in meeting the practical demands of school teaching. This paper aims to assess pre-service Physics teachers’ competency in designing blended lesson plans before training and to pilot a competency assessment instrument. To develop the questionnaire, the eDelphi method was used to identify three levels of performance for the behavioral indicators of this competency, and the survey instrument was then constructed on the basis of these levels. The questionnaire was administered to students via Google Forms together with descriptions of the behavioral indicator levels. The survey results at Hanoi Pedagogical University 2 showed that 74.2% of the submitted responses were valid. Among these, 61.0% of students were classified at Level 1 (Basic), 34.7% at Level 2 (Proficient), and 4.3% at Level 3 (Advanced) in terms of their competency in designing blended lesson plans. These initial findings indicate that the assessment instrument is clear and reliable. At the same time, the study identified students’ competency levels prior to training, thereby confirming the need to foster competency in designing blended lesson plans and to further develop the training program so that pre-service Physics teachers can effectively acquire this capability.