What is the term for being marked by a blithe unconcern?

The term "insouciant" specifically means showing a casual lack of concern or indifference, often in a way that suggests a carefree or relaxed attitude towards serious matters. This aligns perfectly with the definition of being marked by a blithe unconcern, as it embodies a sense of nonchalance and ease.

While "indifferent" indicates a lack of interest or concern, it doesn't quite capture the light-hearted and carefree quality that "insouciant" conveys. "Unperturbed" suggests a state of calmness or stability in the face of potential disturbance, but it lacks the carefree attitude that is central to the definition. "Carefree" suggests a lack of worry or responsibility, but does not inherently imply a degree of unconcern that is blithe or light-hearted as "insouciant" does. Thus, "insouciant" is the most accurate term to convey the sense of an untroubled or carefree unconcern.