Watch the video. Then read the review.
I have strictly nothing to say about the Mortal Instruments books, just as I have strictly nothing to say about the Twilight books. I haven't read either and (as my clients will attest) I don't give a tinker's cuss about originality. I only care about whether a book works.
Disclaimers out of the way, I intend to make a serious point about communicating with your audience. We'll see if I succeed.
I was looking for a movie to go see with the boy. We're pretty catholic in our tastes, so I look at the trailers of pretty much whatever is on at the nearest multiplex (Saint Dizier). I think we'll probably go see the new Cruiser vehicle. Since the absurd (but not wholly dreadful) Knight and Day (2010) and the even less dreadful Tropic Thunder (2008) where the presence of the consistently terrible Mr Stiller is more than mitigated by Messrs Black, Downey Jnr, Nolte and indeed, Cruise, I have discovered that I actually don't hate Cruise any more. Feel free to miss Knight and Day, but if you haven't seen Tropic Thunder, rent it even if you hate Stiller as much as I do. Which is a lot.
Excuse my digression. So I came across the trailer above, and was so impressed with how little I wanted to see the movie that I decided to do a little research which resulted in (the above disclaimer, and) the discovery of the Guardian review linked above. This is actually a review of the trailer, and is, I think, intended to be "light-hearted". Poor fool. Does not know not to mess with teen fandom. And here is a movie made from a book by a writer who started out writing fanfic, and who has a significant following as a fanfic author.
I rather enjoyed the review, as much for what the comparison with the Twiglets says about the Twiglets, as for what it says about this movie. But the Guardian review of the trailer of a teen movie made from a series of teen "dark, urban, edgy teen fantasy" written by an author of fanfic (about whom there may or may not be a pre-existing controversy) is entirely by the by.
Go back, watch the trailer again.
That is the essence of a great trailer, which is also the essence of a great blurb. Less than 10 seconds into the trailer I knew beyond doubt that I wasn't going to go see the movie. Not even if the boy wanted to. (He doesn't.)
A good movie trailer makes you think: "hmm... I might check that out." But only a great movie trailer tells you in no uncertain terms "man, this movie ain't for you."
And that is exactly what a great query letter does. And that is exactly what a great blurb does.
In your Blurb you DO NOT WANT to convince everyone to try your book. If you succeed at that, you know what you're going to get: one star reviews from all the folks who hated it. Who were disappointed. Who feel you failed to deliver on your promises. Who wanted a romance. Who wanted a thriller. Who wanted a dark, urban, edgy teen fantasy.
If you get your blurb exactly right, then the ONLY people who read your book will be the ones who love it. Will be the ones who give you gushing five star reviews or sensible four star critiques.
Each line of your blurb should pare away some of the wrong readers, so that the only ones who read the blurb all the way to the last line will be the ones who will love the book.
Here's the blurb to In Our Hands the Stars (1970) by Harry Harrison.
"THE DALETH EFFECT
The key to the stars?
A weapon of mass destruction?
Klein knows he is on to something big when his test bench disintegrates.—Too big to leave in the hands of a nation at war.
But even in a neutral country the terrible secret becomes the focus of
A DESPERATE POWER STRUGGLE–WITH EARTH AS PRIZE"
This blurb excludes the following readers in the following order:
... don't like made up science
... don't like spaceships
... don't like doomsday devices
... don't like scientists as heroes. don't like male scientists as heroes
... don't like national and international scale jeopardy
... don't like having to save the Earth... again.
Of course, I'm not saying this is the only way to do a blurb. Maybe it's a bit too defensive for you. But don't forget, the blurb is supposed to help the potential reader to choose his next book. It isn't supposed to convince him to read it.