Oh, dark feathers when wet isn't dirt, that's just a quirk of feather structure.
There are small air pockets in the feather barbs that scatter light, which makes the colour appear the way it does when it's dry. When the feather gets wet, water enters the air pockets and changes the way the light reflects -- making the feather look much darker.
If you're interested in learning more, read up about structural colouration in feathers. It's pretty neat stuff.