It sounds very much like it is related to the alcohol. How much do you drink? Not only will alcohol affect your liver, but if you are also having stomach pains, excess alcohol can give you gastritis, which is inflammation of the stomach. The inflammation in your stomach could be changing the colour of your stools. However, as the alcohol affects the liver and also metabolises it, bile secretion could also be affected and you could be releasing more bile than necessary which is making your stools darker.
As you said, if you stop the alcohol and your symptoms stop, then why not cut out the alcohol altogether. You are only 17 and have your whole life ahead of you. Why start to ruin it by drinking at an early age?
If you can control this on your own, then all well and good, but if not, maybe try hanging out with different friends who don't drink and think about getting some professional help.