I like it. But I think we need a property to specify how and when the field should be used:

profile: { /* existing profile data */ },
signature: {
    enabled: true,
    condition: 'all' ¦ 'tag' ¦ 'never',
    content: {
      format: "markdown",
      body: "---\n![Logo](https://ipfs.io/ipfs/QmABC...)\n**My Blog** | [Twitter](...) | [Newsletter](...)\n---"
    },
}

The idea is that you can fine tune when the signature is added to a post using the "condition" property. 'all' to add to all (past) posts, 'tag' when the tag is included in the post and 'never' to disable it.