If you don't have a cat box in the house that you puppy could be raiding,and it doesn't sound like you do. Then I would strongly suggest that you give your vet. a call. Has the puppy had a complete check up yet? Along with shots and worming? Even so, this doesn't sound connected to any of that, could it also look like coffee grounds, just not quite as dark? I think you need to call your vet, as soon as you can and talk with them. They will want to exam the little boxer and I think that is best!