Take a look at Fig. 1 (page three) in this PDF: https://www.osapublishing.org/DirectPDFAccess/9295D3CA-C8BD-E5BA-0DBAC4920EEDA3BD_333549/ao-54-35-10559.pdf?da=1

Take a look at Fig. 1 (page three) in this PDF: https://www.osapublishing.org/DirectPDFAccess/9295D3CA-C8BD-E5BA-0DBAC4920EEDA3BD_333549/ao-54-35-10559.pdf?da=1&id=333549&seq=0&mobile=no https://www.osapublishing.org/DirectPDFAccess/9295D3CA-C8BD-... ("Human skin detection in the visible and near infrared"). The photographic process used is only sensitive to blue/near-UV light. People with dark skin absorb lots of blue/near-UV light. This is why their skin looks darker under this process than it actually is. The tattoos also absorb blue/near-UV light, but the (apparently extra-dark) skin masks the difference, as the contrast ratio between skin covered/not covered with a tattoo is lowered.