Writing blurbs for stories and novels is painful for me. I spend longer working out those 250 word blurbs than writing 250 pages of narrative. (Possible exaggeration) However, it's got to be done.

What you need to know about writing a blurb:
1. It's the first thing the reader learns about the story.
2. It introduces the main characters, the situation, and something about the dilemma.
3. It needs to be interesting.
4. Sometimes the blurb can pose a question that makes the reader want to know the answer and hopefully is willing to read long enough to get it somewhere in chapter twelve.
5. The blurb isn't just about the book though. The blurb is about the reader too. Keep in mind who your target audience is and write the blurb for them.
6. Write more than one or two blurbs and select the one that best fits your audience.
7. Limit what they need to know, hold back something for later. Tease their imaginations with enough information that they wonder. Try to get a connection between the reader and the character by the end of the blurb.