Titan Blood is used for some weapon upgrades after you unlock every weapon. I unlocked a couple things, but don't really like one of the things I picked. Is there a way to recover this spent blood, or do I just have to live with my choices and try to get more for other weapons that I like more?
As of the current version (1.0) there is no way of recovering titan blood once its spent on an upgrade.
Luckily they're rather easy to get as you work your way up the pact of punishment you can earn 2 per weapon. They also occasionally show up at the store in Act 4.
You can also trade Ambrosia(or 15 keys during a limited time offer) for them at the Wretched Broker.