Dark, black or tarry stool in a dog should be treated as a reason to call your vet today. It can mean digested blood from bleeding higher in the gut. A recipe change is one possible benign cause among several serious ones, so it is not the assumption to start from, however recently you switched foods.
Why this particular change is handled differently
Most stool changes after a diet switch are unremarkable. Softer stool, more of it, a lighter brown, a bit more gas: these track the fiber, fat and moisture in the new food and settle within a week or two.
Darkening is the exception, and the reason is specific. Blood that enters the digestive tract high up, in the stomach or small intestine, is partly digested on its way through. Digested blood is black. By the time it appears, it no longer looks like blood at all, which is exactly why it gets misread as a food effect.
So the sequence that feels reassuring, new food on Monday and darker stool on Wednesday, is not evidence of anything. A dog can start bleeding internally in the same week you changed its dinner, and the timing rules nothing in or out.
This article will not tell you what is wrong with your dog. It will tell you what to look at and how quickly to act.
Melena, and what it looks like
The veterinary term for stool containing digested blood is melena. The classic description is black and tarry, with a sticky consistency and often a distinctly foul smell different from ordinary stool. It can look like wet coffee grounds or like road tar.
Fresh red blood on the outside of a stool is a different finding, called haematochezia, and generally points lower down the tract. It is also a reason to contact your vet, but it is not the same sign.
The difficulty is that stool is dark brown anyway, and the line between very dark brown and black is hard to judge in a garden at dusk. Two things help. Smear a little on white tissue, where a black or reddish tinge shows far more clearly than on grass, and photograph it in daylight.
Pale grey or clay coloured stool sits outside this table and is a separate concern that also needs a veterinary call.
The benign explanations, and why they come second
There are genuinely harmless reasons for darker stool, and they are worth knowing so you can report them accurately. They are not worth using to talk yourself out of an appointment.
A diet higher in meat and organ content produces darker stool than a cereal heavy one, because more haem iron passes through. Iron supplements do the same. Medications containing bismuth turn stool black reliably, and owners often give these without thinking of them as medication. Activated charcoal darkens stool considerably too. Strongly pigmented foods such as beet can tint stool, though pigment usually reads red or purple rather than black.
The problem with all of them is the same: none can be told apart from digested blood by looking. Your vet can test for it properly, alongside checking gums, abdomen and blood count. That is a short appointment, and the conditions behind upper gastrointestinal bleeding are ones where hours matter.
What to do now, and what to take with you
Phone the practice rather than waiting to see whether tomorrow is better. Describe the colour, consistency and smell, and say whether your dog is bright or subdued, eating or not, vomiting or not. Mention pale gums, weakness or collapse immediately, because those change the urgency.
Bag a sample if you safely can and bring your daylight photographs. Note anything your dog ate that you did not intend, since scavenging, bones and foreign objects are relevant history.
Bring an accurate account of the diet. Not the brand name alone, but what is actually in the food, when you switched, and how quickly. This is the one place a product page earns its keep in a situation like this: the live listing for the Beef Blend, for instance, shows a single protein grain free recipe of beef and beef liver with sweet potatoes, potatoes, broccoli, spinach, zucchini, celery, green beans and cranberries, and an accurate ingredient list is more useful to a vet than a recalled label.
Detailed figures are not on the My Perfect Pet website. Guaranteed analysis, calorie content, full ingredient declarations and feeding amounts are absent there, appearing instead on the printed pack, in each recipe's Nutrition Facts Sheet PDF, and via the product page weight calculator. Take the pack to the appointment if you have it, and request anything else in writing.
To be completely clear about it: no food, from this brand or any other, is a treatment for dark stool, and changing the diet again is not the right first move. Let the vet look first. General guidance on recognising when a sign needs professional attention is set out in the AVMA's pet owner resources.
After the appointment
If your vet finds no bleeding and attributes the colour to diet, you have bought a clear answer rather than wasted a trip. Ask whether to continue the new food or go back, and follow that rather than a general rule.
If something is found, you will be glad the interval was days rather than weeks. Either way, keep noting stool colour afterwards, because a trend tells you more than a single observation.
Common questions about dark stool in dogs
What does black tarry stool mean in a dog?
Black, tarry, sticky stool is called melena and can indicate blood that entered the digestive tract higher up and was partly digested on the way through. It is not something to monitor at home. Contact your vet the same day, and treat it as an emergency if your dog is also weak, vomiting or has pale gums.
Can a new dog food cause dark stool?
It can. Diets richer in meat and organ content pass more haem iron through the gut and often produce darker stool. The difficulty is that this looks the same as digested blood, so a recent food change is not a reason to skip a veterinary assessment.
How can I tell the difference between dark food related stool and blood?
You cannot reliably tell by eye. Smearing a small amount on white tissue makes a black or reddish tinge easier to see, and a daylight photograph helps your vet, but only a veterinary test can distinguish digested blood from pigment. Assume the more serious explanation until it is ruled out.
Is dark stool in dogs always an emergency?
Not always, but it always warrants a same day phone call. It becomes an immediate emergency if it comes with vomiting, weakness, collapse, pale gums, a swollen abdomen or a dog that will not eat. Waiting to see whether it improves is the main avoidable mistake.
Should I change my dog's food back if its stool went dark?
Not before speaking to your vet. Changing the diet again removes information and can delay a diagnosis. Report what you fed and when, let the vet examine your dog, and make any dietary decision afterwards based on what they find.
Related reading: Why Does My Dog Seem Thirstier After a Protein Change? and Beets for Dogs: Color, Fiber and What to Watch