If you have had GERD
symptoms for a long time , it is possible
that you may have significant inflammation of the lower esophagus
and even ulcers which could bleed. Alcohol can certainly make it worse.
I am glad that you are scheduled to have both upper and lower
endoscopy. Especially, the upper endoscopy
is very important to rule
out ulcers and also a pre-cancerous condition called "Barrett's Esophagus".
If you have dark or black stools, the source is more likely stomach or
esophagus. If you have fresh blood in the stool, then the source is more
likely the large bowel or the colon
Hope this answers your concerns.
I wish you well.