update graphql api docs to clarify blob ref field format

docs/graphql-api.md

···
 
 **Returns:**
 
-- `blob`: A JSON blob object containing:
-  - `ref`: The CID (content identifier) reference for the blob
-  - `mimeType`: The MIME type of the uploaded blob
-  - `size`: The size of the blob in bytes
+- `blob`: A Blob object containing:
+  - `ref` (String): The CID (content identifier) reference for the blob
+  - `mimeType` (String): The MIME type of the uploaded blob
+  - `size` (Int): The size of the blob in bytes
+  - `url` (String): CDN URL for the blob (supports presets)
 
 **Example with Variables:**
 
···
 
 **Usage in Records:**
 
-After uploading a blob, use the returned blob object in your record mutations:
+After uploading a blob, use the returned blob object in your record mutations. You can provide the blob as a complete object with `ref` as a String:
 
 ```graphql
 mutation UpdateProfile($avatar: JSON) {
···
     rkey: "self"
     input: {
       displayName: "My Name"
-      avatar: $avatar  # Use the blob object from uploadBlob
+      avatar: $avatar  # Blob object with ref as String (CID)
     }
   ) {
     uri
     displayName
     avatar {
-      ref
+      ref        # Returns as String (CID)
       mimeType
       size
       url(preset: "avatar")
···
   }
 }
 ```
+
+**Example blob object for mutations:**
+
+```json
+{
+  "ref": "bafyreigbtj4x7ip5legnfznufuopl4sg4knzc2cof6duas4b3q2fy6swua",
+  "mimeType": "image/jpeg",
+  "size": 245678
+}
+```
+
+**Note:** The GraphQL API automatically handles the conversion between the GraphQL format (where `ref` is a String containing the CID) and the AT Protocol format (where `ref` is an object `{$link: "cid"}`). You always work with `ref` as a simple String in GraphQL queries and mutations.
 
 ### Create Records