So, I'm an Asriel Knighter and I recently thought:
"What if the Black Knife is made out of shadow crystals because Asriel's (now lost) dream of being with Dess was just so big you could make a whole weapon out of it?"
The Black Knife may also resemble a baseball bat because memory-thingies of Dess that I don't know how to explain properly. 
(Also, post voting because why not 
)
shadowcrystals are transparent:
'Shadow Crystals, so called because you can only see their shadow.'
the blackshard, and the sword it's a chunk of, are opaque black
'A small chip of extremely hard glass. Oddly, it's nearly opaque.'
they're both weird glass, but they're different kinds of weird glass. they're not interchangeable
The black knife is actually a black plastic butter knife
Can't it be an Obsidian Knife instead? They're renowned for being specially sharp.
– HarbingerKnightTheSecondCan't it be an Obsidian Knife instead? They're renowned for being specially sharp.
– CremBruleeDelta1Peak
Plue...
– Palladium_standardPluest of plue
– KikoPanda