What I don't get is why when faced between the the choice of following their duty to the dragonborn, or slaying dragons, they chose slaying dragons. Even though:
Reports differ widely on the nature of the battle at Pale Pass. But the end result is the same, that the remaining Dragonguard, upon hearing the voice of Reman Cyrodiil, knelt and swore their lives to him, their conqueror and savior. Fragments of from late 1st era texts refer to the warriors dropping to their knees saying "we were not hunting" (or "did not intend", author - rough translation), continuing "we have been searching, for you."
They protected Reman with their lives, as well as his descendants, as the Reman Dynasty ushered in Tamriel's 2nd era. It was through these years that their reach extended, and their order grew to become the Blades. Their conquest of the dragons complete, they only sought to protect the Dragonborn, and through him, the Empire.
...their predecessor's ultimately claimed that dragon slaying was NOT their primary goal, but seeking out the first dragonborn so that they might protect and serve under him.
Well guys, congratulations. You found me - the blade's mission is once again at han- what, you're serious? - You guys are disowning me until I slay
my own mentor in the way of the voice? - How about you guys pack your bags and sod off back to akavir, so commandeth the dragonborn!