A blocked (filled) die would have caused the D to not be there. Isn't this more of a die failure where the center of the D broke off (essentially a cud inside the D)?
Try www.varietyvista.com. I cannot copy the link on my pad. They have some pictures that have the D mint mark filled and indicate it is a double die. You need to have a closer and more detailed photo to see it. So might be an identified error. Take a look and see if yours is the same.
