I just started a new BotW game so I can visit the stables and check the dialogue of each stable employee. Here are my findings:
Blynne says: "I grew up taming horses 'round these parts. I'm confident there's not a rider in Hyrule better'n me." He is located in the Highland Stable, near Fural...
Does the game ever states from what region they are? I know they say they are nomadic people, but frankly, you don't see them outside stables ever. Based on their clothing style that seems to be for cold weather, id put them near the Hebra region. Where would you place their origin?