The blackish grass mouse is a small rodent that uses tunnels beneath leaf litter. A survey in southern Brazil recorded it in deciduous forest with pitfall traps.
The genus Thaptomys is described as small, with short velvety fur, tiny eyes, a relatively short sparsely haired tail, and developed foreclaws.
The FUNASA genus account describes Thaptomys as terrestrial and living in natural tunnels beneath forest leaf litter. A southern Brazilian survey caught this species in pitfall traps.
An extinction-risk assessment has not been added yet.