any idea what that is? Looks almost like black ink leaking.
What iteration of AW is yours? On the May 2017 post you said "for months", so unless you had issues almost immediately it could be possible that you have the 1st generation (so-called Series 0) Apple Watch. I've seen this (yes, it's the water damage) happen on my Series 0 before and it's likely due to the fact that it has weaker water resistance.