Where you turn in the blackthorn artifacts, I have never been to Blackthorn Dungeon and have no clue where the NPC is but he's the dude you wanna talk to lol
Go to a dungeon portal - say Fel Dungeon Blackthorn, it will place you just inside the entrance.
Step outside the dungeon, you are then in Britain guardzone, and your NPC is right next to you.
Very safe and quick, I can assure you.
(just don't be carrying anything important, and insure minax arties in case)
{Edit - Blackthorns castle Felucca is the original simplified version, it is far easier to get in and out of, and find your way around, as there is almost nothing there. Blackthorns Trammel, has had a lot of content added, it's easy to get lost, and takes ages walking around - I prefer the simplified version}.