User blog comment:WitchKingofMorgul/Black Arrow theory/@comment-216.82.243.88-20141025111334

Also even the book version of smaug must have had a resistant under belly, because from my own understanding, almost his entire underbelly was unarmored when he sacked dale and erabor. You mean to tell me entire armies could not land blows to his stomach?

The bottom line was he took out two armies and they had 150 years afterwards to retake an unimaginable amount of wealth and a kingdom and had every reason to do so, yet they didn't because they couldn't. This seems to indicate very clearly that there was indeed something rare about black arrows.