True wet plate photography was only ever sensitive to blue light. So all the blue things looked "brighter" than it's surroundings because colors like green, red, orange, or yellow all appeared dark... because it couldn't register those colors. There's no blue there. So because the color of our skin contains very little blue in it (usually much warmer tones like pinks reds and oranges, no matter how light or dark the skin), we appear darker than normal. This has a particularly interesting effect with people who have red hair and freckles. Wet plate photography will make those freckles really dark and make them pop. It's beautiful.

This is not my image, just a sample I found of a wet plate portrait of a red head with freckles.

Here's a great video about it featuring San Franciscan photographer, Michael Shindler.