I have no idea how well this would work for blades for a mill.  They take lots and lots of stress.

This is the exact design that I am using for the blades that I am building.  What you need to do is create a mold for the foam insert, then poar 3 cores, and glass them.  The molds for the foam need the spar buildup's figured into them.  I would recomend cutting out foam sections for the blades (the exterior shape) in several inch sections.  You can use a blade calculation program to tell you how to cut the sections.  Then create a top and bottom "negative" fiberglass molds from the foam blade you made from the cut out foam sections.  In thoes molds you can lay the fiberglass that you want to use as the spar cap reinforcments.  So when you poar the foam into the molds, they will have the spar cap section already figured in.  Then you can just take thoes foam cores, and lay the spar cap reinforcements, and then the blade skin.

Ususlly the spar caps are made out of a combination of unidirecional fiberglass, and or carbon composite.  The skin should be either a fiberglass mat, or a bidirectional (+/- 45 degree) glass.

have fun and play safe.

If you go the way of foam study commercial methods if you can find them.

Foam, balsa wood, kevlar fabric & epoxy construction - note the stiffening honeycomb of small balsa & foam blocks w/ epoxy coatings bearing the stress ... The black stripes w/ dots are reference points for assembly quality assurance & the black bar is a stress gauge. I assume there is a kevlar structural tube or beam not visible on the surface.