Apple confirms that tattooed wrists will confuse Watch

Apple has stated:

Permanent or temporary changes to your skin, such as some tattoos, can ... impact heart rate sensor performance. The ink, pattern, and saturation of some tattoos can block light from the sensor, making it difficult to get reliable readings.

This technology, while difficult to pronounce, is based on a very simple fact: Blood is red because it reflects red light and absorbs green light. Apple Watch uses green LED lights paired with light‑sensitive photodiodes to detect the amount of blood flowing through your wrist at any given moment. When your heart beats, the blood flow in your wrist — and the green light absorption — is greater. Between beats, it's less. By flashing its LED lights hundreds of times per second, Apple Watch can calculate the number of times the heart beats each minute — your heart rate.

Dark inks, such as red, blue and black, are reportedly more likely to obscure heart rate readings, given how colors play into the device's sensor system.

There has also been some conjecture that the watch may not work as well with people who have darker skin. Hmmm, a bigoted watch.