I haven't had any trouble with shadows in my blades with the 12mm WS2812 5050s. The 8mm SK6812 3535s are only slightly more LED per width (41% for 5050 vs 43% for 3535), but significantly less along the length (50% for 3535s compared to 28% of total gap). Although 12mm is almost half of the available 23mm inside a thin wall 1" blade, where as 8mm is about 1/3. Depending on the diffusion and blade type either strip or gap shadowing may seem more prominent.
Perhaps more importantly, I wonder what the brightness difference is between the 5050s and the 3535s and how that affects shadowing. Datasheets are next to useless, this seems like a job for Space Windu's gadget.