Hi,
I probably would have your dog checked over if she is not eating and is a bit down. The hot weather can have quite an effect on a dog's appetite and exercise level but not so much that it stops altogether.
The black faeces sounds a bit like it may be and indication of some bleeding in the stomach or small intestine. Obviously blood is normally red in colour but once it has been digested on the way through the guts, and the proteins have been broken down, it tends to then turn black - and it is usually mixed through the faeces, giving it a generally black appearance, It is important to make sure that the blood loss is not the reason for your dog being poorly, as they can lose very variable amounts of blod through the gastrointestinal system depending on the nature of the problem.
You vet will hopefully do a blood count to check that the blood cell numbers are high enough and that the blood clotting is normal, and then perhaps an xray or endoscope to check out the stomac and intestines.
I don't want to worry you unduly -and if it is very simple then you may just need some antepsin treatment, but as she is poorly I would get her checked for the more serious problems.
I hope this helps,
Jen
Dog Veterinarian
BVetMed MRCVS since 2004, have a high caseload of canine patients and perform acupuncture.