Persistance paid off. I figured out why. I'm not entirely sure this was the only thing causing it, but it seems the mesh I was editing in nifskope, needed to retain its original name, and instead, I needed to rename the mesh from the horse body I was importing. Thusly Blaze.nif stayed the same, and I renamed horse.nif.00 to Blaze.nif, then copied the branch over and hit 'apply'.