Three days in advance: Make 22 petunias. Use tip 103 and royal icing to make petunias in 1 5/8 in. lily nail lined with aluminum foil. Add tip 16 star centers. Cut pearl stamens and add 5 to each flower. Make extra flowers to allow for breakage; let dry.
Ice 2-layer cakes smooth with buttercream icing. Tint fondant blue and roll 1/8 in. thick. Use largest oval cutter to cut 93 oval medallions. Attach medallions to cake sides approximately 1/8 in. apart. Attach medallions to tops of cake in a circle 3/4 in. from edge of 8 in. cake, 1 1/4 in. from edge of 10 in. cake and 1 3/4 from edge of 12 in. cake. Use tip 2 and royal icing to pipe 5-petal flowers in centers of medallions and balls around edges.
Dust flowers with brush dipped in Blue Pearl Dust. Mound icing on top centers of cake and cluster flowers. Add tip 352 green leaves. Pipe tip 4 top and bottom bead borders on all tiers.