The color of stones is really just as important as what kind of stone it is. And when you pick a stone, as with any magick, it is important to know how this perticular stone, not the kind, but the individual stone, reacts to your own body chemistry, your feeling about the stone is always more important then the kind, or color. And make sure before buying your stones that you know that what you are thinking your buying, really is what it says it is. Alot of stores sell these magickal semi-precious stones.
Or you could just find stones in nature, preferably in a magickal spot you feel power.
Then, once you buy or find your stones, you must cleanse them. Sometimes they don't need cleansing when you find them in nature, but it's better to be absolutely sure that you don't get any negative energies mixed in with you magick. Or even if the other energies aren't negative, it's best to be sure that when you charge a stone, that's all it's charged with, it works best that way, and you can be sure that your magick is done purely.
I'v read many different methods for cleansing, so whatever has worked for you before, or what you trust will work, or whatever you'v read is best for whatever certain thing your planning to use your stone for, you should do.

Blue Stones:
Healing, harmony, understanding, journeys or moves, peace, calm emotions, stopping nightmares, restfull sleep, purification of inner soul.
The color blue is by neptune, accoding to Cunningham, and by jupiter according to others. Examples: Lapis lazuli, labrodorite, sapphire, azurite, fluorite, blue quartz, sodalite, chalcedony.

Lapis lazuli:
A water/venus stone. It's expensive, and very powerful.
The journeys or moves that it helps to create wil benifit you on all levels of you being. It is not a stone to be used lightly. The purpose of any ritual in which it is used will be raised to a higher, more spiritual virbration.

Labradorite:
Is a stone of jupiter and gives hamony and healing.

Sapphire:
A gem of the moon and water, has been long consideed a very powerful, potent enhance to magick , especially star sappires.

Azurite:
It increses psychic powers and prophetic dreaming.
Its vibrations are especially easy for beginners to handle. On a spiritual level it cleases and purifies. But not so deep as to be disturbing.

Sodalite:
Meditiaon, and to gain wisdom.

Blue Fluortie:
Stregnthen power of other stones, also adding it's calmness and clearing thinking to any situations.

Blue Quartz:
Is receptive stone, giving off vibrations of peace and tranquiltiy.