Apple has confirmed on a support page on its website that dark wrist tattoos have the potential of interfering with the Apple Watch's heart rate sensor (via Trusted Reviews). It was reported earlier this week that some tattoos, particularly dark or saturated ones, can affect the Apple Watch's heart rate sensor and ability to register contact with the wearer's skin.
"Permanent or temporary changes to your skin, such as some tattoos, can also 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."
Apple does not offer a useful solution beyond connecting the Apple Watch wirelessly to external heart rate monitors such as Bluetooth chest straps, suggesting that users with tattoos covering the wrist may be forced to deal with this issue. Apple outlines skin perfusion and rhythmic movements as two other factors that can affect the performance of the Apple Watch's heart rate sensor.