The assessment of teaching quality in blended learning modalities has become a key element in the context of higher education. However, current evaluation systems present certain limitations. Behavioral scales overcome many of these limitations, offering an alternative for this task. This study describes the process of constructing an assessment instrument with behavioral scales to evaluate university teachers in blended learning modalities, following the BARS (Behavioral Anchored Rating Scales) methodology. The design process included interviews and surveys involving a total of 477 students, as well as a panel of professors who were experts in this teaching modality. The behavioral scales in the final instrument highlight the importance of certain particularly significant teaching-related aspects of blended learning models, namely: teacher-student communication; learning resources; course design; and the teacher’s technical competencies. The authors conclude that the final instrument provides clear and unambiguous feedback, enables the teacher to take specific corrective measures, and reinforces the formative purpose of evaluation in these modalities.