Minecraft ignores the namespaces for custom blocks. This can cause incompatibilities between add-ons, and allows custom block definition files to modify vanilla blocks in a way that does not seem intended.

Multiple blocks with the same name

If multiple blocks have the same name, but different namespaces, Minecraft will only load one of the blocks, and the rest will be unavailable.

Example: Custom blocks defined in behavior packs with the following identifiers:

Addon A:

"identifier": "addon_A:test_block"

Addon B:

"identifier": "addon_B:test_block"

Minecraft ignores the namespaces (i.e. "addon_A" and "addon_B") and considers these two definitions to be the same block. Only one of the custom blocks will be available in Minecraft.

Modifying vanilla blocks

Attempting to modify vanilla blocks by creating a custom block json file with an identifier like this:

"identifier": "minecraft:grass"

results in an error message that indicates that the "minecraft" namespace cannot be used for custom blocks.

However, vanilla blocks can be modified via behavior pack definition files by changing the namespace but leaving the name the same:

{
    "format_version": "1.12.0",
    "minecraft:block": {
        "description": {
            "identifier": "a:grass"
        },
        "components": {
            "minecraft:loot": {
                "table": "loot_tables/blocks/new_loot.json"
            }
        }
    }
}

This will cause all vanilla grass blocks to use the new_loot.json loot table instead of the vanilla loot table. This seems to be unintended.