a blade could be made of many rather unusual ... things
first of all ... sound. A plane of sound projected from a hilt could be terrible if it touched unprotected flesh, but also crystalline beings would be in dire peril. Perhaps a siren or similar being (harpy?bard?) would have to give up his life or at least voice to create it... or have its voice taken, of course.
then, the idea of light would not have to be obsolete... while a blade of light is unable to parry (except through destroying the opponent's weapon, it is the same as the angelic sword of fire, but capable of working underwater. Clerics of sun deities, aasimar and celestials could have some ... or a powerful sun mage could make one, of course. If you included a fine-tuning button, you could de-focus it and turn it infra-red or microwave for cooking, and also make it ultra-violet for tanning (or ultra-violeNt against vampires
A chainsword made of vampire teeth sure would do SERIOUS damage, as well as draining the life of victims, while one of bullette teeth could cut through armor like paper.
then, a blade of nothing
would also serve the user well - say, a static disintegrate spell.
Like there is the 'chromatic wall' wall in DnD, you could make a chromatic blade, smaller, but more durable.
A demon with fiery blood could have a sword fuelled by his own vitae
It could be damaging, like the water sword, but drain the user, but then, don't give it to Alien, please!
These are my ramblings so far