Why do people think Jagar Tharn an Imperial? As far as I can tell, that idea came with Online as the Tharns there are Nibenese nobility, but Jagar himself is only referred to like http://en.uesp.net/wiki/Lore:The_Real_Barenziah,_v_5, when it comes to his lineage: "I just don't trust that mongrel Elf. Part Dark Elf, part High Elf, and part the gods only know what. All the worst qualities of all his combined bloods, I'll warrant." He snorted. "No one knows much about him. Claims he was born in southern Valenwood, of a Wood Elven mother. Seems to have been everywhere since -- "
Barenziah, sunk in the contentment and lassitude of pregnancy, had only been humoring Symmachus thus far. But now she suddenly dropped her needlework and looked at him. Something had piqued her interest. "Symmachus. Could this Jagar Tharn have been the Nightingale, disguised?"
Symmachus thought this over before replying. "Nay, my love. Human blood seems to be the one missing component in Tharn's ancestry."
With the addition of Online to the lore, it is likely that Jagar did actually have human ancestry (or took the name for himself), but he was definitely identifiable as an elf.