There are so many causes of GERD, you just have to go through the tests to determine what is wrong. Stool color have everything to do with speed food goes through the bowel, I would let the docs deal with that and not worry. An easy answer would to take Immodium, very effective, but do talk to your doctor.
Hi and welcome to the GERD forum.
It is possible to have reflux but not have heart burn.....so,that could explain why you do not feel the burning....
You mentioned you got the DX....but what else has your Dr offered you in the way of combating...this condition and symptoms?