I don't know much about this in a formal sense, but as a reader I can tell you what my personal expectation is.
I expect to determine a few quick things from the back blurb:
Who is the protagonist? Male or female? What is the genre and tone? Is this fantasy, scifi, fiction, some obscure sub-genre? Does the voice/tone already bug me in the blurb? Is there a question to be answered or something to be discovered in this book? Are they alluding to something that I want to know more about? Is there a mystery or something to be resolved? Do I want to know how they did it? I also look for fun elements that may not be the main point but fun, like timetravel or pet Dragons or mining asteroids or something about the world.
A few of my pet-peeves:
I hate it when the whole blurb is just quotes comparing this book to that or what its won. That tells me nothing! I don't care what important people are saying about it- I can find that information elsewhere.
I will put the book right back on the shelf if I feel they told me too much. Did they write a summary and already tell me the ending? I'm not interested in a journey if I already know what the finale is. Unless you convince me the journey itself is worthy of intense scrutiny. Soul searching type stuff.
I also don't like preach-y stuff. Don't tell me this book will change my life or it's a new classic. I'll decide that for myself you just tell me what I'm going to be reading about.