Hi,
Blackguards seem to be caught within a very fine line of what they can and cannot do in terms of builds (sword and board or 2h builds). Their nemesis the paladin on the other hand has received some very nice paths to choose from.
I know we are not 5th edition, but there template for the Blackguard has a couple of Paths: The shadoweaver and the Dread Knight which have some interesting ideas.
They use souls taken in battle to power some unique abilities also, having features like this tied to paths for the Blackguard could breath some life into the Blackguard class that is sorely missing in my personal opinion. I would flesh this out but i sense this would be something the DM team would need to work on themselves.
Lets bring back the Blackguard as a fearsome adversary! 
Thanks for reading