The blood would’ve represented the blood of the victims who were taken by the Infinity Stones (notably the Power Stone) and Thanos would’ve walked across it as he approached young Gamora in Soul World.
We’re learning far more about Avengers: Infinity War now that the home release is upon us.
However, as DeLeeuw explained, the blood made the sequences “a bit dark,” so the decision was made to move away from that look.
In a recent interview with Slashfilm, VFX Supervisor Dan DeLeeuw revealed that Thanos was originally going to walk across a river of blood after snapping his fingers.