Getting Bright Blood in is the best way to continue to progress your game. There are a fair number of ways to do so, but they are not all as intuitive as some of the others.
The game is in Early Access, so there may still be more ways to get Bright Blood as the game continues to grow and expand. The Early Access version of is still being improved as the game continues to get updated.
There will be more enemies, more characters, and more upgrades as the game gets closer to full launch. For now, however, players will have to figure some pieces out for themselves, including how to use Bright Blood to progress.
Your character in will be dependent on you collecting the Bright Blood in order to progress. There will be a number of different ways to do so, and there are several different skill trees and stores to keep track of in the game.
Right now, there are a few known ways to get Bright Blood. The best and most fruitful way to get Bright Blood is to kill enemies.