From the neuroimaging data, we conclude that we cannot define the content of our thoughts during "mind blanking" because our inner speech system does not work at that time

The neural correlates of “mind blanking”: When the mind goes away. Toshikazu Kawagoe  Keiichi Onoda  Shuhei Yamaguchi. Human Brain Mapping, August 7 2019. https://doi.org/10.1002/hbm.24748

Abstract: Mind blanking (MB) is the state where our minds are seemingly “nowhere,” and attention calls no perceptual input into conscious awareness. It is little investigated, perhaps partly because it is difficult to detect the mysterious periods of blanking. In this study, we found that our participants could intentionally produce a state of MB whose neural correlates were deactivation of Broca's area and parts of the default mode network (namely, the hippocampus) which would be active during mind wandering (MW), in addition to activity in another region in the default mode network (namely, anterior cingulate cortex). Because the behavioral finding replicates a previous report of ours, we suggest that the simple instructions that we used to induce MB should be effective. From the neuroimaging data, we conclude that we cannot define the content of our thoughts during MB because our inner speech system does not work at that time. Another possibility is that we actually think of nothing in the MB state. Although more sophisticated studies would be needed to uncover the mechanism of such a phenomenon, the present study provides a methodology and clues for understanding MB and related concepts such as MW, awareness, and metacognitive ability.