I noticed what I thought to be black ink on a few of my fingernails.
The black lines ran vertically on my nails. Spent forever scrubbing trying to get the "ink" off. It wasn't coming off, so I decided to google black lines on fingernails....it wasn't ink.
It turned out to be splinter hemorrhages under the nails...it is called that because it sort of looks like splinters unless you are a clueless fibromite who thinks it's ink. It is associated with some of my other health stuff. I called my rheumy and will see him, next Tuesday...this is a first for this symptom.
I had to laugh though, thinking it was ink. 
Oh well my hands are real clean. 
Hope you have a good evening. 
Hugs, Robin