Thanks for replying.
I'll start off by saying this condition has already gone from to worse in that a small breed like a Yorkie loosing body fluids from both ends for 3 days is quite a big deal. While some dogs can get just stomach upsets that may be treated at home easily, a small dog should start that treatment pretty quickly; they get dehydrated quickly. The good news is that her gums seem to have normal color, which I'm happy to hear, because in saying her stool is black, I'm worried about her bleeding from her small intestines. Black stools often signify old blood (travelled longer, originating from stomach or small intestines), and while sometimes this can happen with intestinal parasites, ulcers, and just the continuation of diarrhea, a small dog can lose too much too fast. If at all possible, I would get to a vet today with her. Having one or the other every 2 hours implies to me she will not be keeping anything in her nutrition-wise or hydration-wise as well.
I do not at all suggest waiting on this, but I understand in some situations you have no choice. If that is the case, I would at least try fasting for 12 hours from any food (if she is eating) or waiting 12 hrs since she last ate, and then see if she'd keep down some pepcid at a dose of 0.25mg per pound of body weight, as long as your Yorkie has been fairly healthy otherwise (no liver/kidney disease or on heart medications). Then you can try very very small amounts of a bland food, like Gerber baby food (chicken or turkey flavors are good) or boiled chicken. Just about a 1/2 tsp or a couple of licks.
If able to hold down, try again in an hour this same amt. If doing ok, do this for a few hrs, then about 1/2 a meal amount in 12 hours from the start of this. You can repeat the pepcid in 12 hours as well, if you feel it is needed (while the bland diet helps the most, the pepcid may help speed along the GI's settling).
If doing ok, keep on bland diet for a couple of days, the gradually switch back to normal food. If not able to hold down even the pepcid, she'll need a vet visit asap, as injectable medications will be called for, fluid therapy, and further diagnostics may be needed to see if this is more than just a stomach upset and if she is indeed bleeding for a more severe reason. Often antibiotics are needed for the bacteria that has built up in the GI tract. Also, make sure she's drinking water or even some unflavored pedialyte the entire time in extremely small amounts at a time, and keep hiher away from heat and indoors while recuperating, except for short leash walks only to do her business.
Again, I strongly suggest a vet sooner than later on this. If this black indeed is blood in the stool, there is one condition known as HGE, hemorraghic gastroenteritis, which if not treated quickly can lead to very quick death. Dehydration itself can put a little dog into near fatal conditions. I wish you both the best of luck, and don't hesitate to reply if I can help further.