There are two different chemistries of blue LED available; the gallium nitride variety, and the silicon carbide variety.
The silicon carbide blue LED was the first commercially produced. The beam color on these is more of a cobalt blue, and fairly dim. Average light output on one of these units would be around 200 metercandles in a focused beam.
You are more likely to find the gallium nitride variety used in flashlights, as the light output from a gallium nitride blue LED is about ten times as intense.
The physical structure of the two different types of LED is different; a silicon carbide LED usually has a black die, with a wirebond connecting in the center. The die itself is conductive, allowing the attachment between the die and the die cup to form the other contact.
A gallium nitride LED's die is actually formed on top of a small sheet of laboratory-grown sapphire, which is nonconductive and requires two wirebonds for its electrical feeding. If you look at the structure of one of these, you will see the two wires attached at the corners. A white LED uses a gallium nitride blue chip covered in a phosphor that emits a wide spectrum of light colors ("white" light) when bombarded with photons.
If you want to find out just about everything there is to know about LEDs, check out the Internet's Virtual LED Museum, located at http://ledmuseum.home.att.net