The wet plate collodion process is only sensitive to blue/UV light. Those tattoos must fall elsewhere in the spectrum.

As an aside, I've just started doing them myself and it's clear why there's been a resurgence. Scans don't really do them justice, and the aforementioned sensitivity makes freckles/other features really stand out. It's also really hard, and takes a good 5-10 minutes per shot.

Take a look at Fig. 1 (page three) in this PDF: 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.