Big Blue
Well I am sure that those B.A.D.D. boys will be here soon with some excellent tips but in the meantime here are a few ideas I have.
What encounter level are you looking for?
I would think @7?
That would be a
Juvenile
According to the MM
Size "L"
'melee' would use:
a bite, 2 claws, 2 wings, and a tail slap. It has a 10 ft reach.
breath weapon....Line @80 ft.
Special abilities now includes
Sound imitation
Has
Caster level 1. (can also cast cleric spells and those from Air, Evil, and Law Domains as arcane spells).
Described as "vain and territorial"
Stats include an
14 INT and
15 Wis.
What is the setting/area?
It says they like to burrow into
sand to cover their
Blue coloring.
Also, they "love to soar in the hot desert sky and use their coloration to their advantage."
"They prefer to eat herd animals such as camels,..Which they cook with their lightning breath.
Prefer to attack from above or burrow beneath the sand until opponents are 100 ft away. Blue dragons on run from a fight if the are severly damaged as they view retreat cowardly."
If this dragon is a juvenile one might think it feels like it has something to prove and is trying even harder to hold its area.
Good spells could include:
Alarm for its lair,
Endure elements in case those pesky spell casters havew some nice fireball type spells.
Shield to stop those pesky Magic Missiles.
Chiil touch maybe?
Ray of enfeeblement should make things tough/interesting.
To set the mood if they are in the right environement I would:
In a desert have them come accross a small herd of camels that have been partially cooked and eaten, as if by some large wild beast.
Lots of carnage!! Body parts scattered about!
More destruction than is necesary for just a simple meal.
If it is a more temperate area Horses or some kind of cattle/bison would work as well.
Let the dragon use its
Sound Imitation[I/] to mess with the party a bit. Sounds of a trapped and hurt animal or a human child or baby, but when they get there, there is nothing to be found. Try to get them spooked.
Use some cantrips at night.
Some Ghost sounds and Dancing lights maybe hit somebody with a Ray of Frost, especially a sleeping spell caster.
Maybe cantrip a Light into their area at night if they think they are being sneaky.
Let it stock them and put them on edge.
The first Real attack could come in the morning or whenever the spall casters are preping their spells.
Blue could do a flyby attack and light up a few characters with his lightning a bit and get them well shaken up!
Now that they know what is going on it is ok to really take it to them. If Blue has any spells to cast on itself use them and go in for a physical assault. Give them its full on armory of 'slaps and smacks'.
Get a character in a bite and breath that lighning at the same time.
You can hold back and try not kill anybody yet.
It's ok to beat them up really good and then leave. Let them see to their wounded and spend some resources getting ready for the next time and try to keep them on edge.
Big Blue knows it's better than they are and, as a Dragon, is The apex-predator!
A 15 HD dragon gets 4 feats(?) you could even bump it one more HD for a few more hitpoints and pick up a 5th feat...
Good ones to keep him up on the party could be
Improved Intiative so you an put them on the defensive.
Sunder could be impressive.
Flyby attack (go see Reign of Fire if you haven't already..lots of Flyby attacking there 
)
If any of the party can fly Wingover would be good to show them some real manueverablity.
The Hover feat is great to kick up a cloud of debris, really mess some more with those spell casters!!
You never have to kill a character if you don't want to.
Remember that this particular kind of dragon is especially Vain and a dragon's vanity is Supposed to be a Major Weakness.
It doesn't feel the need to take them out and underestimates their power when they finally do get organised.
But first,
stalk them, scare them, rough them up and awe them with its horrible power.
Then, if they get it together, have a huge finale battle and let it go out in a blaze of glory.
The whole time Blue is thinking there is just no way these pitiful adventurers could ever pose a real threat?!
Well I hope some of that will be a help and maybe it will give you some good ideas?
Happy hunting!
BADD boys BADD boys..what'cha gonna do...what'cah gonna when Big Blue comes for you!
