This mixed-method quasi-experimental study examined whether metaconceptual grammar teaching impacts on (a) students’ L1 grammatical understanding, (b) their ‘blind’ use of grammatical concepts and (c) their preference of using explicit grammatical concepts over everyday concepts in explaining grammatical problems. Previous research, involving single group pre-postintervention designs, found positive effects for metaconceptual interventions on secondary school students’ grammatical reasoning ability, although a negative side effect seemed to be that some students started using grammatical concepts ‘blindly’ (i.e., in an inaccurate way). While there are thus important clues that metaconceptual grammar teaching may lead to increased grammatical understanding, there is a great need for more robust empirical research. The current study, involving 196 Dutch 14-year old pre-university students, is a methodological improvement of previous work, adopting a switching replications design. Bayesian multivariate analyses indicate medium to large effects from the metaconceptual intervention on students’ grammatical understanding. The study found a similar effect of the intervention on students’ ability to use explicit grammatical concepts over everyday concepts in tackling grammatical problems. No evidence for increased ‘blind’ concept use as a negative byproduct of the intervention was found. Additional qualitative analyses of in-intervention tasks provided further evidence for the effectiveness of metaconceptual interventions, and seemed to indicate that cases of blind concept use, rather than being a negative side effect, might actually be part of a gradual process of students’ growing understanding of grammatical (meta)concepts. We discuss these findings in relation to previous work and conclude that linguistic metaconcepts can improve L1 grammatical understanding.