What makes a blues song blues?

Ciao,

I hope the question is clear - what are those things that makes a blues song in fact party of the genre 'blues'?

I mean, I know about the Twelve-bars scheme, but I also know that there are blues songs which are more flexible in their progression.

So, besides the cases where the progression falls into said scheme, what are those things that makes blues 'in-arguably' blues?

Also, can a progression like the following be considered blues? - * | i / VImaj7 | V7 | III | II7 | *