Blue

When you combine black with dilution (dd on the D-locus) all eumelanin will be diluted to dark blue rather than black. 
Blue dogs vary from a fairly pale grey to such dark grey they look black unless seen in a certain light. Their nose leather is a dark grey colour, and the eyes are amber or brown. Blue can appear in any pattern, such as blue brindle, diluted blue merle (often referred to as slate merle) etc.

There is a condition tied to the dilution gene named Color-Dilution Alopecia, or CDA for short. It is believed to be the same condition that is known as Black Hair Follicular Dysplasia in some breeds, and most commonly seen in blue dogs. Symptoms vary greatly but include loss of hair, wrinkles skin, allergies and dermatitis. Some dogs can be born without symptoms but develop them later. 
Due to the conditions tied with the gene, it is of utmost importance that breeders are aware of the potential risk and breed responsibly.

There is one exception as far as blue is concerned in the dog world. Research has shown that the afghan hound does not have the common blue dilution that is seen in other breeds. It appears to have "its own" mutation instead to produce blue.

A blue and tan blue lacy. If one looks closely, one can see how the nose leather and lips are dark blue rather than black. The tan however has been unaffected by the dilution and is as red as in a black and tan dog. The blue shading (rather than black) can however give a lighter appearance.

A dark blue great dane

A toy fox terrier that is a noticably lighter blue shade than the great dane or even the lacy above. It is not known what causes differente shades in different dogs. Blue is a very rare colour in the toy fox terrier, so Ellie here is quite unusual for her breed.

Octavia is a blue sable greyhound. This is referred to as blue fawn in many breeds.