I'm pretty sure you can do this multiple times but just like in Origins you can use the Ice Staff to shoot the Flaming Carriages and extinguish the flames once you do, a zombie blood will spawn behind PaP:
Locations are all in the blue portal and you'll need to shoot the ice staff:
- Behind the generator
- In the far right wing from where you spawn take a right by the generator and look on the cliff side.
- In the left wing when you spawn into the PaP room, go far left. This one is a bit tucked in the distance. Look to the left of the Trench ledge