We call immature or eclipsed drakes "dirty drakes". We do a lot of dirty drake mallards for northern states, canada, and New Zealand. New Zealand has a lot of pacific black and mallard hybrids that look a lot like early season mallards with dark bodies.
I've actually seen very few hunters in my region who actually decoy divers, it's almost all pass shooting. Most hunt from boats and just get under the flight path and bang away. I hunt on foot with a good retriever and need to put the birds in close in areas they really didn't intend to go. When it comes to divers most people use only drakes and do ok, but as most people also know, birds behave different from region to region and what works in one place doesn't necessarily work in others. We spent years watching bufflehead fly right on by our decoys which consisted of both goldeneye and bufflehead drakes, in the first seven years in that particular spot we decoyed one drake and one hen, it was pretty frustrating. We would see several dozen breeze by at about 50 yards on every hunt, but they just wouldn't come in. Everyone was telling us you only need drake decoys, but one friend back east said I needed to add hen decoys, so I did. It was like flipping a switch when we added hen bufflehead decoys, nearly every bufflehead we saw came in, landed or got shot at. The difference was night and day. I usually run 6 hen and 3 drake bufflehead decoys to have a mix of what we are generally seeing in our stretch of river. Divers are slow to mature and more often than not the groups of birds we see in our spot are hen heavy, or at least they appear that way at a distance. This past season we decided to try the same thing on goldeneye decoys and made a 50/50 spread of drakes as well as hens/dirty drakes. The season before we had done some counts on drake to hen ratios in goldeneye and it was far more hens than drakes. This past season when we added immature drakes and more hens our decoying ratio went up. Dirty drakes in divers probably aren't any different to a duck than the hen decoys, but I'm always trying for a more realistic set up with fewer decoys. We run only a dozen decoys or less, but try to make sure to keep the numbers even between the hens and drakes. An immature goldeneye drake will have a black bill, brown head and anywhere from a hint of a white patch on his face to a full blown white patch. I also believe that on a fully mature goldeneye drake decoy it's the black that is more important than the white when it comes to visibility.
Lots of guys will tell you to only use drakes for divers and it does probably work in their stretch of water, but it doesn't work where I hunt very well and didn't work at all for our buffleheads. I haven't noticed any difference in using dirty drakes for mallards in my area, but the guys in north dakota that have used them swear by them in early season.