From Left To Right- (1-2) Sky Blue, Normal (3-4) Cobalt Blue, Normal (5-6) Mauve, Normal
Dark Factors In Blue: Blue budgies come in 3 different shades or dark factors, sky, cobalt and mauve. The belly, chest and rump will usually be the places to show the body color, with the exception of opaline budgies and those that can mask mutations such as ino, df spangle, lacewing etc where the body color will be masked.
Sky Blue- (the lightest blue shade) is the color of a white based budgie with 0 dark factors Cobalt Blue- (the medium blue shade), is the color of a white based budgie with 1 dark factor. Mauve- (the darkest blue shade) is the color of a white based budgie with 2 dark factors.
Cheek Patches And Main Tail: Depth of blue in tail and cheek patches will vary with sky, cobalt and mauve as you can see in the above link to tail and cheek patch photos.
Body Color Suffusion: The body color should not be on the head or wing markings of any mutation (except for opaline) of a well marked budgie.