Now that understand that the blurb's job is to convince a customer to buy our book. We now have to construct the second most important part of your passive marketing strategy. The book's cover is the most important part of selling your book. If a customer doesn't click on your thumbnail sized cover, they will never see your blurb. Your blurb won't even get a chance to do it's job.
Step one: Identify your main character. Identify your plot/conflict. Identify your setting. But SB Jones, I have more than one main character. No you don't. One of these characters is reacting, growing and changing as a character in reaction to the other. That is your main character.
Many authors can't identify or describe their own books. It's like asking a parent who has a 30 year old son/daughter to describe them in a few short sentences. Do you talk about them in broad strokes or try and cram in all the minute details of their life? Remember part one? We need to get customers to click the buy now button. If your describing your son to someone in hopes of seeing them settle down, taking about how they wet the bed isn't going to help.
If you still struggle with what the blurb is supposed to do. Hop over to YouTube and watch the movie trailers for this year's blockbusters. These are exactly what your blurb is trying to do. Get people excited about your book so they have to click the buy now button.
Step two: Outline and structure. We need to take a few minutes and look at some technical aspects of your blurb. Amazon and other retail websites limit the display area for blurbs. Amazon keeps it down to 4000 characters, but what you really need to watch out for is number of lines. If you have a large five paragraph blurb with four blank lines in-between. Your blurb will get cut off and a customer will have to click a link to expand out the blurb. Anytime you put a click between the customer and the buy now button. You WILL lose sales. Some retailers only give you a line or two before they cut it off, so you really need to remember this.
This link is a collection of good blurb writing articles. When it comes to plotting your blurb. You need to setup your characters, your conflicts and your setting. Like the movie trailers, often they will start with the main character talking while at the same time they are giving you the setting. Vin Diesel's scratch voice, while you see fast cars, big explosions and crazy stunts. Then it shifts to the conflict and you see other stars, quipy one liners, more explosions and rocking music until at the end of two and a half minutes you are standing with a fist full of dollars yelling "Take my money!"
That's what your blurb has to do.
At minimum. You should have a character, conflict blurb structure. Other structures include character, character, conflict. Or Setting, character, conflict. There are any number of workable paragraph structures for blurbs. The genera is also important. What works well for a Star Wars blurb isn't the best for a Rom-Com.
My personal preference is a Character Conflict structure and I weave in the setting. I also leverage the most important marketing point of my book in my blurbs: the cover.