Depends exactly what your current route is. I’m making the assumption that you go over the top of Bolt’s Law from Edmundbyers to Rookhope? There is an ‘off-road’ track runs through the old lead mine past Rookhope, and comes out at the ruined cottage just before the descent into Allenheads. It’s fireroad type stuff, so nothing majorly exciting, but it’s quite pretty!
Then the obvious thing to do would be to cut across Hexamshire Common, either before or after Allenheads and head towards Slaley Forest, and take the Byway back into Blanchland. Either take the scenic route around the reservoir, or climb back up to near Bolt’s Law and take the fun off-road route back into Edmundbyers.
That’s a fairly decent ride out, almost all off-road, and there’s a few routes to explore around Hexamshire common.
Not sure about heading over to Killhope. Would be interested myself if there’s any hidden gems.