Valfaris

All pistol weapons, except 1*, follow the same upgrade scheme: 3 blood metal, then 6 blood metal, then 7 blood metal AND a Blood of Valfaris for their final upgrade.
The *exception is a late-game weapon called Bishop's Reach which requires 7 blood metal for first upgrade then 9 blood metal + 1 Blood of Valfaris for its final upgrade.

So fully upgrading any "pistol" weapon will require a total of 16 blood metal, and 1 Blood of Valfaris an item which doesn't appear in the game until partway through the 8th stage.

These are the applicable pistols/primary weapons:
Plasma Pistol (starting gun)
Wolflight
Vroll's Sidearm
Embryon
Therion's Call
Bishop's Reach
The Knife