An all or mostly black watercolor tattoo would be a new item to me. "Watercolor tattoos" don't necessarily fade any faster than more traditional styles. The broader discussion about how they will wear over time is the "issue", and has mostly centered around how over time all tattoo ink spreads or "bleeds" in the skin over time - and how the lack of a black outline may result in that eventual spreading making a watercolor styled piece become indiscernable.
This is discussion is mute if the "artist" isn't capable of or just doesn't get the ink where it belongs, in which case it's going to fade anyway.
I'd sure like to see an example of what you're describing. Can you post a link to the artist's work, or do you have a picture of her drawing for you?