The supporting document does clarify that the ink, pattern and saturation of the tattoo do have the ability to block the light from the sensor, which then makes life difficult for Apple Watch wearers to obtain reliable readings. In other words, those who happen to have darker tattoos which happen to cover more of the skin’s surface will potentially run into far more complications as opposed to those who have lighter tattoos, not to mention being smaller in size.
There you have it, sometimes there are sacrifices that need to be made in the name of fashion. Well, I suppose that one could live without the functionality of keeping track of your heart rate and still make use of other connectivity options of the Apple Watch if one is already tattooed.