The irony. It's not a cat staring at an invisible supernatural creature. It's a supernatural creature staring at an invisible cat.
Human doesn't notice either way obviously. Thinks that they're just staring into space
2 reasons..
When blood leaves the body, it darkens due to rusting.
The ground isn't made of white sand. When that imagery is presented, It's to describe trampled brown dirt. That kind of surface makes liquid substances mixed into it darker.