User blog comment:FairyRaven/Fairy Tail Chapter 423: Do It All for Love, Review and Commentary/@comment-18654001-20150309111355/@comment-9595671-20150309135630

To give another anime as an example of Natsu's strength, I'd like to point out One Piece's time skip.

A Pacifista that had given the Straw Hats trouble 2 years prior, when Luffy, Zoro and Sanji fought two more Pacifistas they each only used one attack to destroy both of them.

So as B214 says the fact he got stronger and one-shot Bluenote is actually logical, especially when you remember before the timeskip none of the slayers beat Dragons much weaker than the ones who taught them and he needs to have become stronger to be able to have any chance against Acnologia.

Plus after the Tenrou Island incident Bluenote was one of few villains who we never heard from again, not every villain becomes a former villain in the series and Bluenote's one of them. So its actually logical to show Natsu's new level of strength by facing an enemy that he originally wasn't able to defeat on his own.