Apple is acknowledging that the Apple Watch's heart rate monitor, which uses light to read blood flow under the skin, has issues with some tattoos . iMore's own testing found that the Apple Watch does indeed have difficulty getting heart rate information through tattoos with dark or solid colors. Apple's support page relating to the heart rate monitor on the watch has been updated to reflect the issue.

From Apple:

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.

The Apple Watch's heart rate sensor uses different spectrums of light to track blood flow through the user's skin. For those experiencing problems between their tattoos and the watch, know that you can turn off wrist detection in the Apple Watch app on your iPhone, which will prevent the device from automatically locking, though this will disable the use of Apple Pay.

Source: Apple

Apple Watch