It can be both, because the definition of blithe is “showing a casual and cheerful indifference considered to be callous or improper.” And that is negative, but the other definition is “happy or joyous.” Which is positive.
Answer: Its probably a tense effect.
In "Cranes," just after Song-sam lets his friend escape, he hears a shot.
This is the moment when he becomes very worried about his friend, because he starts thinking that maybe that bullet ended up in his friend, hurting, or possibly killing him. He wants to find out what happened which is why he ventures forth.
