Honestly that's a tough call - you'd have to be really good at staying out of the broadsword range with blade lock. But I'd have to say Doom because of the pain immunity and thrust boost, plus the cursed fire increases the damage percentage. The cursed fire and venom might get you with Blightfang, but like I said you'd have to know how to blade lock well enough.. it really comes down to skill. I've won against it before using Doom, though.
Wow im glad everyone didnt choose blightfang because its a broadsword with high attack and they actually chose the sensible root with doom rattler, i personally would prefer doomrattler the thrust boost is amazing it alone can almost kill pretty much every enemy if you put enough into it
Now, let me explain my decision. First off is attack. 95 beats 77 any day, aside from the fact that Blightfang is a broadsword (can damage multiple enemies at once). They both have the special skill Cursed Fire, but Blightfang has Rank 3 while Doom Rattler has Rank 2. The third is secondary skills/attributes: Doom Rattler has Pain Immunity and Thrust boost, whereas Blightfang has Venom Strike and Critical Strike. Critical Strike is a very good skill, giving you bonus damage against the target; Venom Strike is also good. Pain Immunity is good too, as is the Thrust boost, so this was the most difficult for me.
Doom Rattler seems like a very good weapon, but in the end, Blightfang Edge is the better weapon IMHO.