MG - if the bleeding occurs higher in the intestinal tract/stomach, it turns it black when it goes through the digestion process. Whenever Aidyn has big nosebleeds he has very dark black tarry poos from all the blood he has swallowed.
The only other thing I can think of that hasn't been suggested, is that Nurofen 'can' cause intestinal bleeding (would be rare though) as it is apparently quite harsh on the stomach, which is why it needs to be taken with food. However that sounds pretty extreme and I doubt it is that! Probably much more likely to be diet related.

However if it was only a once off in Abbey's nappy I wouldn't be too concerned... just keep an eye out, as I know you are doing!