Red applies a slow, and an active nocturne always has the slow.

Bloodthirster does not apply a slow.

When i say bloodthirster, i always mean with full bloodthirster stacks. With full stacks, it has 25% lifesteal, and if you are relying upon the spellvamp from unspeakable horror, I have no euphemism for this, its just stupid. The active applies a slow, and 300 damage, with bloodthirster, you'll fulfill that purpose in one auto attack.
Paranoia builds off of AD, no argument.
I believe that paying a extra gold for an item like this on NOCTURNE is horrible, and is just bad.

Gunblade offers the same attack damage as the Bloodthirster, plus more lifesteal and spell vamp. Don't forget the active also does damage, and has only a 60 second cooldown. Pretty nice to have if red buff is not available.

Most importantly though, it is the only item in the build that gives AP, which makes Paranoia much more effective.

Personally I believe all of this is worth paying an extra 625 gold for instead of getting another Bloodthirster.

Switch out the gunblade for another bloodthirster or something.

Gunblade is super expensive considering that this is an ad build, with youmuus, you want to milk the attack speed as much as possible by stacking massive attack damage.

The gunblade offers practically the same lifesteal effect as another bloodthirster.

Getting ap for your unspeakable horror is terrible, since you will do a negligible 50 more damage to your target with it, as opposed to getting bloodthirster and doing more damage with an auto attack.

Getting gunblade for the active is redundant, since you can fulfill that effect by grabbing red buff. If nocturne is played, and farmed right, if you get the chance to use the active for the slow, he will decimate an overextended the same opponent with unspeakable horror to stop them, and duskbringer to get close to them.