User blog comment:FairyRaven/Fairy Tail Chapter 423: Do It All for Love, Review and Commentary/@comment-27.32.207.84-20150310075440

The last time Natsu and Bluenote met, Natsu still only had one dragon element, no second origin, and probably hadnt had the added drive of seeing his guild nearly die, not to mention witnessing Igneel die. Tartarus was a traumatic time for all of them, the scars will run deep. You would have to assume at the very least, as Natsu does, he probably went overboard on the training. Hopefully we might get an OVA or short Film showing his training. On a slightly less serious point, judging by how Bluenote struggled to remember him, maybe he suffered amnesia at the hands of Gildarts Empyrean. So he was laid up in rehab for part of those 8 years.