Here are some results of my research and your description:
Possible cause: Lentigo
Lentigo is a flat brown to black spot (several spots ~plural : lentigines) that can affect the skin and lips. It may be associated with long term sun exposure (solar lentigines, also known as actinic lentigines or liver spots) or with systemic diseases (non-solar lentigines) like Peutz-Jeghers syndrome.
Peutz-Jeghers syndrome is a genetic condition characterized by the presence of gastrointestinal polyps (read more on intestinal polyposis syndromes) and hyperpigmentation of the skin and mucosal membranes. This condition is associated with a very high risk of intestinal cancer. The darkened skin patches (lentigines) is in most cases accompanied by darkening of the mucosal lining in the mouth (buccal mucosa).
Source for more answers: healthhype dot com
What Causes Black Lips
Frequent drinking of hot beverages that contain caffeine like coffee and any kind of hot tea.
Constant biting and licking of the lips.
Iron levels of the body can contribute in making the lips black. When there is an increase or overflow of iron levels in the body, this will usually show in the lip area and can be observed when it turns black.
Low vitamin B levels. (There are a lot of B vitamins!)
Change in hormonal levels that are contributed by age, infection, diseases, and other factors.
Certain toothpaste brands and lip medications along with other kinds of medications used to treat diseases and infection.
Improper dental fixtures can also cause lips to darken but very rarely.
Allergic reactions and injuries.
Source for more answers: simple-remedies dot com
I can't let this pass without saying something: "Used to wake up thirsty though I drink a ton of water." If you no longer do this, great. Being thirsty often can indicate diabetes. Just had to mention it.
I'll search further if you have more input to give me.