Feature
Being able to use different color blend modes, as seen here: https://en.wikipedia.org/wiki/Blend_modes
From my experience, the most useful ones would be normal, add, screen, multiply and overlay.

Potential uses
This would mostly be useful for more advanced techniques, such as masking certain sky elements, or getting a specific color output.
For example, the screen mode gives much softer blending, compared to the blending the vanilla game offers.

Details
These new modes could replace/rearrange some of the currently implemented properties entries.

For example, the currently undocumented blend property could be re-used as a new Blend Entry containing one, from the list of valid blend types.
The is_solid property in combinations with blend produces the same result as the normal blend mode, when both are set to true, which would make is_solid obsolete.
The currently used blending mode could be named as vanilla.

In short, is_solid could be removed, and blend would use a format like this:
"blend": "vanilla"
"blend": "screen"