It sounds easy, but as you pointed out, it takes some artsy stuff to get the pattern on there. But, I would photograph my sword, manipulate the image in photoshop to be black and white and warp the shape to the katana blade's shape. Then print it out at actual size - luckily I have a printer at work that prints on 2" wide rolls of paper. Use the print out as a stencil for the film, then cut out the design.

Wow, that makes it sound like a lot of work. This project is going to keep me busy all next summer.