I noticed that my nightblade does both shadow and neutral damage. How does this work?
Split damage weapons
When a monster is vulnerable to a damage type, it incurs 20% or 25% more damage than usual. When a monster is resistant to a damage type, it incurs 75% to 80% less damage than usual. So let's redo Facejuances' calculation.
If your sword wants to do 50 normal damage and 50 shadow damage, then it will 100 damage to constructs and beasts, maybe 112 damage to slimes and gremlins, and maybe 62 damage to fiends and undead.
Check out the damage table on the Acheron wiki page, to get more information about how the numbers change. (I suggest Acheron because there may not be a damage table for Nightblade.)
Does 50% normal 50% shadow damage.
Anything weak to shadow it will do 50% bonus damage instead of 100% bonus damage.
a pure shadow weapon would hit 200% on something weak to shadow
a half shadow half normal would hit 150% on something weak to shadow
Not the exact bonuses of bonus damage but you get the idea
which is why flourish is best (can i get a pure elemental/shadow sword plz?)
That is how damage should work. Check the wiki data and you'll see a Flourish actually has a similar damage bonus to an Acheron. The penalty though is ~80% vs 40%.
I wrote a long piece quite a while ago about why this might happen. I don't know if this is the way it actually works or not, but haven't heard better theories:
http://forums.spiralknights.com/en/node/9955#comment-57112
I don't have the right numbers but I'll put an example: lets say the total damage is 100 (50 shadow 50 normal)
To an enemy that is neutral to shadow you'll do just 100 damage (no bonus)
to enemies that are weak to shadow, yould do--> 50 from normal + 75 for shadow(because of the bonus)
to enemies resistant to shadow, you'll do 50 from normal + 25 from shadow (because of penalty)