It’s too bad that darkness and shadow blade are both concentration spells, since they work really well together mechanically and flavorfully.

There’s not really a ton that interacts with shadow blade in particular, especially for Warlocks. Easy answer is to use green-flame/booming blade with it to make up some for not getting Extra Attack. You can also dual wield with it since it’s Light, so you could be a Bladelock with another Light weapon as your Pact Weapon and do that.