The .25% chance calculation assumes the blade has a 1/9 chance to drop. From previous sample sizes we know it doesn't. The grimoire drops like crazy, on the other hand.

Still, regardless of the exact math, I think we can all agree killing a boss 50+ times and having nothing to show for it is not fun gameplay, especially when along the way you get stacks of another classes weapon, even when they aren't there. My first two runs were harpoons we greeded because there weren't any dragoons present.