I've been thinking, why do the Elemental brandishes get all the fun? We've got three (count 'em!) lines of Elemental Brandishes, all of which inflict some flavor of painful status effect on anything they hit. What does Shadow damage in the Brandish lines? A single sword line, no status effects at all.
So, in order to a) increase the prevalence of Shadow damage based weapons and b) even out the Brandish lines, I suggest:
Change the Nightblade line (Nightblade, Silent Nightblade, Acheron) to have a slightly lower charge attack damage. What's the reason for this? It now also inflicts Strong Poison to any monsters hit by the charge attack, in the same way that the Elemental Brandishes inflict Strong [status].
Add two more Brandish lines, both dealing Shadow damage to enemies.
New Brandish line A (Dazeblade, Stunning Dazeblade, Stupefier) would inflict, as the name suggests, Strong Stun. Hopefully, OOO will resolve all these nasty Stun issues before they introduce more things that inflict Stun.
New Brandish line B (Hexblade, Jinxed Hexblade, Nemesis) would have an extended charge time, and (can you see where this is going?) have a charge attack that has a 25% change to inflict Strong Curse to any enemy hit by it and a 12.5% change to inflict Strong Curse on the user. As an additional balancing measure, perhaps this particular line would need to have its recipies purchased from Sullivan in the same way that the Voltedge line currently does.
I'm aware that the names are a tad uninventive, but I'm kinda awful at actually generating usable titles for things ::P