Misty Blue has a white (non-pearly) body with blue hair. Her Cutie Mark (symbol) is three two-tone bluebells, in front of light blue swirls. Her heart (right hoof) is cornflower blue. She has lavender eyes with a light yellow highlight and light blue dots.
Dream Valley linkThis is what I found at Dream Valley. As for information we have on her here...I don't she was adopted and s/he is not listed in our Adoption Listings, which means the pony was newer than our last update of that listing.
S/he can have family, but they would be an adopted family made up of other arrivals. You could pick some from the long list or just say that since she was old enough decided to live on her own.
There is no back story for her, you get to make it up! That's the fun part. You may want to take a look at some other approved G3s to get an idea of what a finished character profile looks like.
Mostly, you want to choose the age she arrived, how long she's been in Rainbow Dreams, where she chose to live, where she lives now, what occupation she has and any hobbies she may like.
This is really where you get to be creative with your character, yes you're given the look of the pony but that's pretty much it. The rest is just as if you were creating a G1 or other character from scratch.
I'd suggest that since Rainbow Dreams is sort of slow, that you have her current home (if not her home from the beginning) be Ponyville. Any occupation you would find in our world is pretty much available for her as well. Maybe she's a jewelry store owner, a pet shop owner, a groomer, a baker, or even a school teacher. Once you being play you'll make friends and be able to add them to her profile, like I did with Kiwi Tart when Water Fire met her.
She could also work in the Castle if you'd like. Its really quite open to any possibility.
I think this will give you a good source of information to finish her up but if you need anymore help feel free to let me know!