Charcoal biscuits for dogs

Penny had a black biscuit last week and over the next two days her poo was blue. I'm wondering if I was correct in thinking the biscuit was the cause of the strange faeces.

I've been surfing the Net to see what info there is about charcoal biscuits and here's some of what I found:
At Springer Rescue they discussed copophagy (eating faeces) and said: 


This is a condition where your dog eats his own poo! Not always easy to correct, but many people say give the dog more charcoal biscuits in his diet, also mix pineapple rings into his food, as often when the dog poos he still sees it as food due to not being properly digested and therefore the pineapple being very acidic, may well put him off!

Penny hasn't touched her own poo, but I thought she had a nibble at some faeces in the park a couple of times lately. I'm not sure, though.
Wisegeek, in a general overview of the role of charcoal in human and animal health, said

Veterinarians may recommend charcoal dog biscuits to help canine gassiness problems and/or cases of "doggie breath." Dogs should eat only the charcoal biscuits made for dogs and not the kind made for humans. Dogs should eat small rather than large quantities of charcoal biscuits, but veterinarian approval should first be obtained. Charcoal may interfere with a dog's absorption of other nutrients.

Old Mother Hubbard, a manufacturer of dog biscuits, says

Charcoal biscuits really serve three (3) distinct purposes, they help to freshen breath, ease in passing gas, and settle upset stomach

At Yahoo Answers, one reply says

I feed old mother hubbard charcoal biscuits along with evo red meat. I feed the charcoal many hours after he eats the evo so it does not strip away the nutrients from the food. I do this to stop smelly gas, and it works. If you feed dog food in the morning it is best to give the biscuit a couple hours before bed time.

Dogsey – The Site for Dog Lovers, has an interesting list of routine health checks for dogs, and one of the checks says

Observing your dog’s toilet habits may not be among the top 10 spectator sports, but it can provide valuable insight into the health of your dog. The number of bowel movements per day varies considerably from dog to dog. The important thing is that the evacuations are regular and of consistent appearance. Bear in mind that certain foods may change the colour of the faeces, e.g. charcoal biscuits will produce black faeces. Any chronic or acute diarrhoea or constipation requires veterinary attention, as does the presence of blood or mucus. If the urine appears dark, cloudy, or blood tinged, or the dog is urinating excessively or has difficulty in passing urine, again, consult your veterinarian.

It's this last article that has me wondering, because it says faeces would be black if charcoal was eaten. I bought the dog biscuits in an unmarked bag and assumed they had charcoal in them because they are black. But perhaps they are just black-coloured biscuits masquerading as charcoal. As I said in a previous post, I know that black food coloring can be made up of blue dye.

It's still a bit of a mystery but Penny seems to be in good health generally, so I'll have to wait and see...