its normal for skin to darken its pigmentation after trauma and environmental wear.
vixies tummy turned very dark grey/black during her pregnancy, i thought it was dirty but it wouldnt wash off, i asked the vet and they explained that trauma and sometimes jus tlife in general causes the skin on light dogs to blacken. as long as theres no decaying tissue, open wounds, and its not tender to touch (ie a bruise) its just pigment change.