The plot: In present-day [name_u]America[/name_u], an evil society is trying to destroy everything, and three young teens are the only ones who can stop it.
Character one: [name_f]Ivy[/name_f]–Blond hair, happy, overly-cheerful, naïve, but she learns that not everything is good as the story progresses.
Two–[name_f]Miranda[/name_f]–a girl who has seen way too much and doesn’t trust easily. She has long blond hair.
[name_u]Blake[/name_u]: A boy with dark hair who has a good heart but gives away important information to protect himself.
Since we don’t know anything about these characters other than what they look like, I don’t know.
You have to remember, and this is coming from a professional writer, that it was the parents of your characters who named them – they didn’t spring to teenagerdom, fully-named, like [name_f]Athena[/name_f] from the forehead of [name_m]Zeus[/name_m]. (Unless they named themselves. Then it becomes part of the storyline.)
So who are their parents? Their socio-economic status? Their religion? Their region? Their language? Their politics? Their ethnic backgrounds? All of this factors into what a parent names his/her child.
Once you flesh out your characters, then you will know if their names fit. You have to do the work yourself. We can’t do it for you, because they are, after all, your characters.
I definitely think [name_f]Ivy[/name_f] and [name_f]Miranda[/name_f] go together. Not so much [name_u]Blake[/name_u]; it seems a bit bland with the other names. Maybe a J name would be better like [name_u]James[/name_u] or [name_m]Jonah[/name_m]?
In contrast to what other posters have said, I don’t think your names absolutely need to go, since (I’m guessing) they’re not related. I think miloowen made some excellent points. Before you name the character, I think you need to know where the character comes from more than anything else.
Funny, though, I always picture an [name_f]Ivy[/name_f] as a redhead, a [name_f]Miranda[/name_f] as a really dark brunette, and a [name_u]Blake[/name_u] as a blond! So, in that respect, they don’t fit for me. 
They certainly fit the time period!