Silver and Blue are variations on Black.
I think the best way to think about it (but not a scientifically accurate way) is that Blue comes from one dose of the fading allele laid on top of Black and the color is lightened to a gun metal grey color. Silver gets a double dose and the Black lightens to a sterling or platinum color.
There is discrepancy at the edges of each color..... is light blue actually dark silver? Also, far far too many people call their bad faded Blacks, Blue. Also dogs that get all greyed out from grissleing are sometimes called Blue. I don't think this is a true Blue or Silver color.
If you want a good Silver or Blue, the best thing is to look for a puppy who has a very light face at 8 weeks. If the face is platinum, the puppy will most likely be silver. If the face is a lighter dusky grey, the dog will probably be blue. If you want good color, you probably want to pass on the puppy that has a face that is dark and dusky but not really black. These dogs often end up with that odd middle ground between bad black and very dark blue. I think this is one of the least attractive colors in poodles.
Belle is one of the nicest blues I've ever seen. She really does have a "blueish" tint to her color:
http://www.calaespetresort.com/showdogs.html
I think Jib is a good example of a true Silver. The clearing process takes time, but you can see how light Jib's skin is. Plus, I had to post his page so that you all could feel guilty about the fact that you are just not getting out and doing enough with your own dogs
http://www.geocities.com/jibandsuzicope/