I was having this problem, darkening or brown to black spot at the lower part of my mouth to my chin. It is sometimes itchy. I did an observation while brushing by not letting the water that comes from my mouth with the tooth paste but rather spitting it directly to the sink. after a few days of doing this i observed some improvement till the dark spot eventually removed. Hope this post can help some of those who may encounter the same problem.
Thanks,
VAL3131