Does gerd cause black stool? or is this a sign of something more serious?

No: GERD does not cause black stools. But if you have taken Peptobismol, it can turn stool black. If you didn't take that, then you will need to see your primary care doctor soon. But if you are having several black stools and feeling lightheaded or dizzy, then you will need to go to an emergency room as it maybe a sign that you are bleeding in the bowels.