I'm still not 100% with the Draugr being followers of the Dragon Cult, I understand it's a retcon but I'd have expected an explanation for all the Draugr you find in more recent tombs, did the process of making Draugr pass down from the dragon priests into general Nordic use?

Draugr are "cursed" undead followers of the Dragon Cults, according to in-game lore. However, the game uses the "draugr" objects for all general decomposing/zombified bodies instead of creating separate entries because there's relatively few of the latter... it's easier to test and reuse what you already have all over the place than to create something new that needs to be kept track of.

I do have to wonder how they tie in with Bloodmoon's draugr, though. According to that game, draugr are Skaal/Nordic warriors who turned to cannibalism (a crime against nature, so they were cursed with undeath by the All-Maker). Should it be taken as a mere coincidence that the Skaal use the name "draugr" for undead warrior cannibals, while Skyrim Nords use the same name for undead Dragon worshippers? Or is there a link somewhere between cannibalism and dragon worship?

Draugr could just be a Nordic term for cursed undead.

I think it could be from the fact that they are enchanted to permanantly be animated. I'd do that with a soul gem, and since Oblivion they have all been blue. Also, when they are killed, it is because taking damage drains the soul gem's regenerative abilities, and it ceases to function.

Well, it is Old Norse for "Ghost".

On an unrelated note, Death in the Terry Pratchett mini-series has blue eyes.