Question

I was installing your mod and created the example step by step this way Block Overlay Example

So. This is the result.

assets/example/textures/sand_overlay.png.mcmeta
assets/example/textures/sand_overlay.png
assets/minecraft/models/block/gravel.json

Any texture must be added to the block atlas in order to be used on blocks. Did you add the example:sand_overlay to the block atlas?
For context:
#34 (comment)
#34 (comment)

Perfect. Thanks a lot.

Maybe you can edit this at the wiki page Block Overlay Example