Aww, sorry I just spotted this Ellie. I didn't get a chance to get on today. I think you might have hit the nail on the head though. Black tarry poop is always a sign of intestinal bleeding somewhere and if you suspect an ulcer then you could be right . I know dogs can be put on meds same as us for ulcers, my neighbour's dog is on Tagamet right now and seems to be doing okay...of course he's a collie so much bigger than little Myndie. But I'm sure the vet can put her on something for her size.
Glad you have that vet appointment, will be holding my breath until you let us know what's what and of course you will both be in my prayers. Don't go blaming yourself, I've taken the blame for being blind for what happened to my dogs but it doesn't help. We all give over 100% to our dogs so it was nothing you did or didn't do.