Just a thought...have you used aspirin or other pain medication for your dog's arthritis? Or any herbal treatment? If so, that could possibly be causing this symptom.
Pooping blood is not a good sign, that's right. It could be something serious. On the other hand it could be something not quite so serious. For instance, if a dog has had a bout of diarrhea (caused by anything, even if caused by just 'eating the wrong thing') and keeps straining, like they often do with diarrhea, that in itself can cause small amounts of blood in the stool, even though the actual cause may not be a deathly serious thing.
Eating chocolate (very bad idea) can cause this.
The only way to know however, is to take your dog to the vet promptly. Do take a stool sample with you.
There may well be some herbs that will help eventually, when it is discovered exactly what the cause of this is. But I would not advise using herbal medicine right now. The vet needs to make a diagnosis, and recommend a course of treatment straight away, and herbs would interfere with this treatment. Also, depending on what is wrong, herbal medicine may or may not be appropriate. I wouldn't advise using them without advice from a herbal vet anyway, as some herbs safe for humans, can be toxic for dogs.
This is a Medical Emergency!!!! Herbs take too long to work & she/he could bleed to DEATH in the mean time.....Please, call your Vet NOW & get your dog taken care of!!!!!!