Background <p>Mind blanking (MB) describes a state in which a person reports an absence of thought, sensation, or awareness. Although thought sampling methods (TSMs) are widely used to capture MB during sustained attention tasks, concerns remain about whether individuals’ reports of MB during the task reflect genuine blankness or failures in memory retrieval.</p> Methods <p>We investigated whether adding a “Forgot” option, designed to capture metacognitive uncertainty, to TSM probes improved the interpretability of MB assessments. Japanese participants (<i>n</i> = 183) completed a sustained attention to response task (SART) in a within‑subject crossover study with two probe conditions: one with five common categorical options and one with an added sixth Forgot option.</p> Results <p>The Forgot option was selected infrequently, especially in the first session, yet its inclusion clarified introspective reporting without disrupting behavioral performance (hit response time, variability, and d′). State frequencies showed moderate-to-high session‑to‑session stability, indicating reliable individual reporting tendencies. A correlation analysis revealed the expected associations between the participants’ reports of mind-wandering (MW) and questionnaire-based trait MW scores across both conditions. Critically, MB reports correlated with trait MB scores only when the Forgot option was available, suggesting that the additional option reduced misclassification of retrieval‑based uncertainty as MB.</p> Conclusions <p>Incorporating a Forgot option enhances the construct validity of MB measurement by distinguishing genuine experiential absence from memory‑based uncertainty, while preserving task performance and the stability of state reports. This refinement improves the precision of TSM‑based assessments and offers a practical tool for advancing research on cognitive absence and attention–memory interactions.</p>