well.. after givin' it a Rhino pass I've seen that ALL normals are in the right way...
I've loaded the aarose model into poser ans also have that problem, i give no mat to the rose and still ALL the back sides were black, when rendered with no shadows it looks right.
Just in sake of completion I have a try in Daz Studio(3) and have no problem, it looked right in preview ans in render so I think it have something to do with the way Poser handle bakfacing polys...maybe if you separate the petal mat group in let's name it "layers" like "petals1... petalsn"
it might help. If you want I can send you the file with subgroups made...

hope that helps(someway...)