With the rapid advancement of technology and the need for flexible learning solutions, educational institutions are integrating more and more digital components into their curricula. Blended learning (BL) has emerged as a promising approach, combining the strengths of traditional classroom instruction with the advantages of online learning. This mixed-methods study investigates the effectiveness of BL in an EFL undergraduate course on students' English proficiency and through their attitudes. In the second semester of the academic year 2022, 269 students at a university in the west of Thailand participated in BL for a course named "Basic English I." Data were collected via online pre-and post-tests, questionnaires, and semi-structured interviews. Of the 269 participants, 194 students from 13 majors completed the pre-and post-tests and questionnaires, and 20 volunteers were interviewed for in-depth information. The paired samples t-test and Cohen's d indicated significant improvement in students' English proficiency post-intervention, demonstrating the effectiveness of BL. The questionnaire analysis also revealed positive attitudes towards BL, while a thematic analysis of the interviews highlighted BL's flexibility, accessibility, and ability to enhance engagement and understanding. Participants suggested improvements to the instructional methods and technical support as well, in order to further enhance the learning experience. [Note: The page range (320-351) shown on the PDF is incorrect. The correct page range is 320-352.]