I'll give my thoughts and how I would personally rewrite each if they were my own stories. Of course it's all just suggestion and you don't have to adhere to any of it.
For the first one: Asking direct questions to the reader comes off a bit too casual for me. It might be appropriate for children 10 and under books though. So I would cut the first two lines out. For the rest it works okay, I would just change the tense and wording slightly.
"Natalie Austin wasn’t expecting to fall in love with Dave Park.
But a change of feelings sparks quickly when she catches him singing her favorite song while playing his guitar."
"Set against the backdrop of urban St. Louis, this (possibly insert an adjective here?) teen struggles to accept her growing love for Dave.
Despite that, she fights to keep him in her life." < But I have questions about this part. Why is she conflicted with her feelings over him? How does she fight to keep him in her life?
"Will she follow her heart to stay and listen to him finish his song as the Missouri sun sets?" (I've always been a little iffy on questions like 'how will they reach their goal?' at the end of blurbs. They serve their purpose fine, but it's kind of unimaginative.
For the second: The scene could be set a bit better and more succinctly.
"On a hot summer day, Adore met a charming freckled boy. They whiled away the hours in each other's company until the sun set on their picture-perfect impromptu date (could be worded better but hopefully you get the drift). It was her special summer moment, and Adore was expecting it to be just a fond memory. That was until school started. As fate would have it, Adore sees him again, this time as part of the popular clique she despises. However, she just can't seem to hate him."
And with the last sentence, I'm not fond of when blurbs address the reader directly, unless it was like a fourth-wall joke.
"A story about two teens from two different worlds and the blooming feelings that bind them."
For the last one: Here, wording is the biggest issue.
"Every two years, Venus Summers visits the beach and meets up with her childhood friends: Kenji Juan, and the siblings Evangeline and Renz. They spend most of their time in the siblings' family mansion where over the [years?days? weeks?] Venus finds herself falling for Renz. Every night onward she wishes upon the stars for requited love. But apparently they seem to have gotten confused as Venus finds out Kenji Juan harbors feelings for her.
To further complicate this summer, while the friends attend a festival, Ocean Shimmers, the lead singer of a featured band, captures Venus' attention. Choices, choices. And Venus thought the school year gave her a headache."
Oh and I would probably read the last one out of all of them. I like reverse harems.