User blog comment:Tyrannoraptor/why use black arrow to shoot Smaug at the weak spot? why not normal arrows to kill Smaug!/@comment-26082545-20150409155640

As told before, a dragon has a VERY high duration and it's impossible to destroy one of those pieces with a normal arrow. It would simply break, while a black arrow was made to kill dragon's. Also, the skin is harder then flesh. Smaug has actually a double defense against, let's say, everything.