This type of "whale head" letter opener that companies give out as promotional items, does a good job of safely concealing the blade.

It works well enough, and it's pure form-follows-function. Nendo, however, wanted to design a blade-hiding letter opener that was also beautiful. The result is their Nautilus Paper Knife:

Even after seeing what's inside…

…I couldn't fathom how it worked. It operates like this:

"When an envelope is passed through the slit on the side, a blade attached to the inner spiral enters under the flap, and opens the envelope. A 3D printer was used to achieve the complex shape of the exterior morphing into the blade, but because of the limited sharpness of the blade, repeated experiments were performed for the cutter's right shape, angle, and distance from the slit."

Also looks pretty sweet in black:

These run ¥13,200 (USD $102) on Nendo's online shop.