minecraft:dirt can be edited as a custom block.

When you put a block file with a identifier  of "minecraft:dirt" in a behavior pack you can edit the dirt block as a custom block. This is not intended behavior as vanilla blocks and items are hardcoded and trigger a content log error when attempting to use them in a behavior pack. This does not happen when using the dirt identifier though.

Below is a example pack and the content log error that is supposed to appear when using a vanilla identifier. To use the example pack give yourself a dirt block and interact with the dirt block.