Some form of urinary tract problem typically causes dark urine in dogs, according to petMD and Hill's Pet Nutrition. Blood in the urine may contribute to its dark color, or crystals may be present that make the dog's urine look foggy.

Urolithiasis, a type of urinary tract disease where crystals or minerals form in the urinary tract, can cause foggy urine and subsequently much discomfort in dogs, explains Hill's Pet Nutrition. Signs of urolithiasis can include straining when urinating, blood in urine, licking of the genital area, incontinence, decreased urine flow, more frequent urination, reduced appetite, and lack of interest in normal activities.

Because high levels of certain minerals, such as magnesium and phosphate, can increase the chance of crystal formation in dog urine, a dog owner must carefully read the ingredients in his pet's food and try to feed his dog food that supports bladder health, notes Hill's Pet Nutrition. Just as with humans, a dog needs to drink enough water, so monitoring a dog's water intake is another important factor in decreasing its chances of getting urolithiasis.

In addition, hematuria may cause dark urine, states petMD. Hematuria causes blood to enter the urine, which in addition to being an indicator of some form of urinary tract issue, may potentially be a sign of a more serious underlying problem. An owner must closely monitor his dog not only when it urinates but in regular day-to-day activities. A dog owner should disclose to the vet any nutritional supplements the dog receives, as this may factor into the diagnosis.