Blurbs are small, quick summaries of your story that are meant to grab a prospective reader's attention and convince them to give your story a chance. In the case of webnovels, where there are so many to read, getting that prospective reader to give you a chance is important.
There are usually three things that get the prospective reader's attention:
Title
Cover
Blurb
The title and cover work together to get the prospective reader's attention. It's what causes them to do a double take. If you can get the reader to do that double take, they will then move on to the blurb. It takes about a second for a reader to decide whether or not reading the blurb is even worth their time.
The blurb itself needs to tell the reader what they can expect to read when they click on that first chapter. At the same time, intrigue them enough to want to click on that first chapter.
This often becomes a balancing act with providing enough information, but not too much that it'll turn off the reader. After all, you don't want to bore them with detail in just the blurb.
The Four Sections
When I write a blurb for my stories, I try to hit on four major points.
The Set-Up - What is the situation, the world, the time period, etc? (2-3 Sentences, Maximum)
What is the setting that the story is taking place in? Think of this as the opening shot to a movie; what do you see? What can you say to describe it at a glance?
You want to give your prospective reader a general idea of where your story takes place. It doesn't require much detail. Hinting at the social structure or mentioning a time period may be good enough.
The Main Character/POV Intro - Who is the character and two or three hints about their personality and background. (4-5 Sentences)
Introduce the prospective reader to your main character.
Pick one personality trait and one physical trait, such as age, and possibly even one life trait, such as occupation, to mention. The gives the prospective reader an idea about your character and how they may react to the situations which they will face.
Let's try an example with Sokka from ATLA (who is my favorite character on the show).
A clever young man left to defend his home against invaders.
What does that say about Sokka: he's likely intelligent, adaptable, and is left with a heavy burden and responsibility.
This also makes you wonder:
Why is he left with such a responsibility? How does that affect him and those around him?
If you have more than one main character or POV, either focus on the absolute main one OR introduce them each with one sentence using a neutral voice. This keeps the reader from placing immediate importance of one over the other. Then, tie them all together implying that they are fated to meet or will be some sort of team or whatever.
The Problem - What is the conflict that is driving this story, but super simplified. (1 Sentence)
What is your protagonist going up against? Is it a specific entity, like a kingdom, or an entire social structure?
Why are they against that? What are the consequences if the conflict is not resolved?
Keep in mind that individual antagonists and villains do not need to be mentioned by name. This is because for the most part, those characters work within a structure or entity that the protagonist is facing. Unless there is one big bad villain is a main character in their own right, they do not need to be individually named.
The Goal - What they want, what they are trying to accomplish, what they are trying to avoid. (1 Sentence)
What does your protagonist want? Do they need to save the world? Save themselves?
Tell the prospective reader what the ultimate goal is so they know what they're rooting for.
The Emotional Notes
Would you be emotionally invested in a character you don't care about? For the most part, it's a nope.
In that tiny blurb, you need to make the reader interested in your character and what random adventure they'll go on. They need to empathize to a certain degree.
You also want to pique their curiosity. This doesn't necessarily mean being mysterious, but just interested enough that they'll want to see where this is going.
Things to Avoid
Do not include too much detail. The reader doesn't need to know the entire tragic backstory of a fallen society. You can explain that to them in the story itself. All they need to know is that the society fell.
Do not write out large paragraphs that are just a giant block of text. Anytime a blurb looks like a massive block of text, it is a turn off to the reader. Keep the sentences short and sweet and spaced out. This makes for easier readability and the prospective reader will be more likely to read the blurb the whole way through.
Do not end the blurb in a question. I know you want to sound mysterious and enticing, but this gives off the feeling that you don't know how it's going to end and are telling the reader "you figure it out'. The question of 'how is the hero going to prevail' is already implied.
Do not make your blurb sound like an advertisement. There is one big thing missing when people do this and that's visuals. Visuals trigger emotions and ideas, so when words are flashed over them, there is much more impact than just the text alone.
Do not try to sound so cool and unique in your blurb. Let that come out in your story, with your writing. I know you don't want to sound generic or boring, but tying to stand out too much in a blurb by using unorthodox descriptions and styles can back fire and confuse your readers. When a reader is confused, you lose them.
In Conclusion
Experiment. You know what you are trying to do: attract readers.
Tell them just enough to give them a taste of your character, their world, and what adventure you're inviting them on.
