Source: Press / apple
Apple Insider explains that the Apple Watch uses a process known asphotoplethysmography to detect that the watch is being worn. By using infrared LED lights the watch detects blood flow under the skin, thus it recognizes that it is on a person. In this case it appears as if the color within the tattoo, is blocking the LED lights from detecting the blood flow.
source:http://www.inkedmag.com/rethink-buying-apple-watch-youre-tattooed/
