So, I'm not sure what I'm missing exactly, but The Blades, who are rooted in Akiviri origins, wear these snake themed armours and architectures. It seems, I assumes, they were men, but it said in the lore that the men of Akavir were eaten long ago by the Vampire Snakes. What exactly were the first blades then? Vampire snakes or men?

The Tsaesci are confusing. In-game resources (Pale Pass ghosts, Alduin's Wall) say they're men, books (though not exceptionally credible ones) say they're serpent-ish (2920, Mysterious Akavir).
What the 'eating' means or symbolizes isn't clear, either.

We have https://www.youtube.com/watch?v=UoXqMg279jc, though.

That would fit interestingly with the 'becoming'.