I use the 3-inch Fusion. First reason I use them has nothing to do with being shield cut. It's because they are lower profile than most other options out there. Second reason is since they're lower profile, I like the shield cut to give more surface area.
If you're using regular Blazers, then you don't need a 4-fletch. The only time you might need a 4-fletch is when using small lower profile vanes. I can't even imagine what 4-fletch Blazers look like.