blossom community storage fallback.

B7.md

@@ -1,7 +1,7 @@
 NIP-B7
 ======
 
-Blossom media
+Blossom Media
 -------------
 
 `draft` `optional`
@@ -39,3 +39,24 @@ Clients may use other facilities exposed by Blossom servers (for example, for ch
 
 [Blossom]: https://github.com/hzrd149/blossom
 [BUD-03]: https://github.com/hzrd149/blossom/blob/master/buds/03.md
+
+### Blob Storage Community Fallback
+
+Kind `4242` is a normal event used to announce that the author has saved one or more blobs from another event on their own Blossom servers. It SHOULD be a reply to the event containing the blobs, using an `e` tag.
+
+The event MUST contain one or more `x` tags with the SHA-256 hashes of the saved blobs, and one or more `server` tags with the URLs of the Blossom servers where the blobs can be found.
+
+```yaml
+{
+  "kind": 4242,
+  "tags": [
+    ["e", "<event-id>", "<optional-relay-url>"],
+    ["x", "<sha256-hash-1>"],
+    ["x", "<sha256-hash-2>"],
+    ["server", "https://blossom.example.com"],
+    ["server", "https://cdn.example.com"]
+  ]
+}
+```
+
+Clients MAY use these events as additional hints when retrieving the tagged blobs. They SHOULD verify each downloaded blob's SHA-256 hash before using it.