Shine in the field is never a "good" thing. If you look at geese heads from a distance you will see some translucent shine, but that is a different kind of shine. Its like a shine off of a mallards head, if you were to put a decoy and a mallard together you would see what I'm talking about. The thing I would worry about more is glare. Glare is something that the bird's heads do not put off, otherwise there would be shiny specks in fields and on water.
However, to answer your question, flat black should work just fine. Although, if your going to do that I would get a flocking kit and flock some of the heads. My water spread is all flat black heads, but my field spread is all flocked heads.
Some will argue that flocking doesn't matter, but when the odds are against you, I would rather have more realistic decoys on my side. I have heard in the old days people would use old tires and stumps for decoys, boy, have we come a long way since those days.
Arise, Kill, and Eat. Acts 10:13
I can do all things through Christ, who strengthens me. Philp. 4:13
I apologize to those I have yet to offend. Wait just a little longer and I will get around to you too!!