When you have mirrored an object using scaling by -1 that object will show it's normals correctly AND will display properly when you turn off 2-sided lighting. However, when you freeze the transforms the normals will flip and the object will turn black OR if the object is part of a group you get the warning that you got and that objects "opposite" attribute gets set but the object normals are still flipped even though they display properly.

So ungroup or separate before freezing transforms.