I’ve never had to officially write a blurb, but these are some things I’ve noticed that blurbs do:
1. Introduce the MC, define their want/story goal, hint at their need
2. Define the setting (e.g., in the Kingdom of Such-and-Such, in a dystopian future, in a world where people live in the sky, etc.)
3. Make the genre clear. I should have no question about the genre after reading a blurb. (This is sort of a pet peeve of mine when amateur authors don’t explain the genre or try to write too many into one story).
4. Hint at the theme. I’ve noticed this often phrased by giving the MC two choices, which sets up thematic conflict between the two or by stating what the MC will sacrifice. Or, it’s just explicitly stated (e.g., …as MC struggles to forget her past as she forges a new future.)
5. This isn’t universal, but I’ve often noticed blurbs will tell you what happens up to the first plot point and hint at the rest.
6. If the story is dual-POV, a separate paragraph introducing each of the MCs is in order and the third tells how their stories will interact.
I hope this is somewhat helpful!
Semper ubi sub ubi.