One: Blurr was, or is, the payment someone ELSE gave to the fae (specifically Mirage) as part of a deal made a long time ago. ex- my firstborn child. Which is why the debts are so absolute

Two: Blurr is the mortal half of a child + changeling pair. And Mirage is the fae, possibly parent, responsible for the changeling half and therefore also the one who "owns" Blurr (I like this theory the most, but it's also the one with the least amount of evidence)

Three: Blurr was a very obvious monster at risk of being killed and gave himself up to the fae. To be paid for at a later date after having a chance to live as a "normal" mech for however long.

To be fair, I'm a pretty big folklore nerd. So I'm pulling hard at some real world older fae stories to supplement these. And there are very specific types of deals that are made where the eventual payment is a person's whole self. Ones where no labor, or marriage, or children are mentioned