The moment the ball of darkness was about to hit his body squarely without even a single thin layer of defense, the scrawny humanoid demon could only brace for impact by gritting his teeth and trying to move his hands to the front of his chest in the hopes that the attack would hit his hand instead of his chest.
Having a broken arm was definitely a better choice compared to having his ribs broken and his insides possibly punctured.
But the ball of darkness, albeit had a lower concentration of darkness element compared to a darkness bullet, only moved slightly slower compared to the darkness bullet spell. It wasn't moving slow enough for the scrawny humanoid demon's late reaction to being able to block it.
As a result, the arm that was on its way to block the ball of darkness couldn't make it in time and the ball of darkness hit the scrawny humanoid demon squarely on his chest.