Textures in Atlases

Hi, I'm trying to understand how to use the new atlases. But the textures don't appear.

minecraft/atlases/blocks.json

{
    "sources": [
        {
            "type": "directory",
            "source": "custom",
            "prefix": "custom/"
        },
    ]
}

#textures minecraft/textures/custom/texture_rewbiqs.png minecraft/textures/custom/texture_aquaheartia.png