I'm not exactly 100% sure, but black stools could mean that there is blood in it. Please have it checked out.

What are you taking for your reflux? It sounds like you need to up your medication or go on some because the chest pain is likely due to reflux getting in your lungs. As for the black stool, i'm not sure what would cause it but if you have some sort of stomach virus, etc it could possibly cause that and an increase in reflux. I would keep an eye on it and see a doctor if it continues for too long.

i am taking omeprazole which is the generic form of prilosec I think. I have been on it so long I just know it by it's generic name.
Do you think internal bleeding clould be the cause for the black stool???

It's possible it could be some bleeding.. Have you ever been tested for an ulcer? Check out this link on the issue:
http://www.Nlm.Nih.Gov/medlineplus/ency/ar ticle/003130.Htm

if it doesn't clear up in a day or two I would go to the doctor and get checked out.