Hum,
The photograph records the ultraviolet light. something our eyes don’t see.
(ie blue bells turn out magenta in photos)
< Doh!, as had already been pointed out >
"That's right, Dougal. You see, ordinary shops sell what look like black socks, but if you look closely, you'll see that they're very, very, very, very, very, very, very dark blue...." - Father Ted