Good morning, that's a great question. It's difficult to be absolutely positive without a better look from an actual exam but there unfortunately are not many possibilities. It is almost certainly from an area at the edge of the porcelain that did not bond fully leaving a tiny place where there is a leak or essentially a space between the porcelain and the tooth behind it. This space may have been temporarily filled by remnants of the cement used that prevented anything from entering but the cement, not fully bonded eventually washed away and has now allowed either a particle or a dark liquid to access that area and stain it. The stain could also occur from your gums bleeding, even if just slightly when you brushed and the blood has seeped into that tiny space and turned dark as blood does over time. The only other option that could have occurred is that a small sliver of the porcelain edge chipped away leaving a small reservoir that is catching stain. You should return to the office where they were placed and have this addressed as soon as possible. If you are unable to go there immediately you may be able to gently brush at the edge with a small amount of baking soda or hyrogen peroxide to get some or all of the stain out. This is not a brushing method you will want to use regularly but in a pinch it wont cause any problems. Good luck.
