As people have mentioned, darker skin can just be pigment, but it is very likely to be irritated skin.

Many dogs suffering from skin-sensitive allergies can develop darker skin with no other signs, sometimes it's just darker of with a bit of flaking. The most common cause is irritation caused by a sinlge flea or chemicals you use on your carpet or flooring, or just scratchy grass or mud on walks.

Staffs tend to lay down flat on their bellies with their back legs splayed like frogs and sometimes drag themselves along like this, so their skin comes into direct contact with the ground and can easily be irritated - that's why aloe vera and other creams can reduce the amount of pigment showing.

Cat