diff --git a/openedx/core/djangoapps/content/search/documents.py b/openedx/core/djangoapps/content/search/documents.py
index ab73123d7572..6a6c920a71db 100644
--- a/openedx/core/djangoapps/content/search/documents.py
+++ b/openedx/core/djangoapps/content/search/documents.py
@@ -28,7 +28,11 @@ class Fields:
     id = "id"
     usage_key = "usage_key"
     type = "type"  # DocType.course_block or DocType.library_block (see below)
-    block_id = "block_id"  # The block_id part of the usage key. Sometimes human-readable, sometimes a random hex ID
+    # The block_id part of the usage key for course or library blocks.
+    # If it's a collection, the collection.key is stored here.
+    # Sometimes human-readable, sometimes a random hex ID
+    # Is only unique within the given context_key.
+    block_id = "block_id"
     display_name = "display_name"
     description = "description"
     modified = "modified"
@@ -54,7 +58,7 @@ class Fields:
     tags_level1 = "level1"
     tags_level2 = "level2"
     tags_level3 = "level3"
-    # List of collection.key strings this object belongs to.
+    # List of collection.block_id strings this object belongs to.
     collections = "collections"
     # The "content" field is a dictionary of arbitrary data, depending on the block_type.
     # It comes from each XBlock's index_dictionary() method (if present) plus some processing.
@@ -339,6 +343,7 @@ def searchable_doc_for_collection(collection) -> dict:
     """
     doc = {
         Fields.id: collection.id,
+        Fields.block_id: collection.key,
         Fields.type: DocType.collection,
         Fields.display_name: collection.title,
         Fields.description: collection.description,
diff --git a/openedx/core/djangoapps/content/search/tests/test_api.py b/openedx/core/djangoapps/content/search/tests/test_api.py
index e4a004635103..3105ad8c007f 100644
--- a/openedx/core/djangoapps/content/search/tests/test_api.py
+++ b/openedx/core/djangoapps/content/search/tests/test_api.py
@@ -186,7 +186,8 @@ def setUp(self):
                 description="my collection description"
             )
         self.collection_dict = {
-            'id': self.collection.id,
+            "id": self.collection.id,
+            "block_id": self.collection.key,
             "type": "collection",
             "display_name": "my_collection",
             "description": "my collection description",
@@ -451,6 +452,7 @@ def test_index_library_block_and_collections(self, mock_meilisearch):
         lib_access, _ = SearchAccess.objects.get_or_create(context_key=self.library.key)
         doc_collection1_created = {
             "id": collection1.id,
+            "block_id": collection1.key,
             "type": "collection",
             "display_name": "Collection 1",
             "description": "First Collection",
@@ -464,6 +466,7 @@ def test_index_library_block_and_collections(self, mock_meilisearch):
         }
         doc_collection2_created = {
             "id": collection2.id,
+            "block_id": collection2.key,
             "type": "collection",
             "display_name": "Collection 2",
             "description": "Second Collection",
@@ -477,6 +480,7 @@ def test_index_library_block_and_collections(self, mock_meilisearch):
         }
         doc_collection2_updated = {
             "id": collection2.id,
+            "block_id": collection2.key,
             "type": "collection",
             "display_name": "Collection 2",
             "description": "Second Collection",
@@ -490,6 +494,7 @@ def test_index_library_block_and_collections(self, mock_meilisearch):
         }
         doc_collection1_updated = {
             "id": collection1.id,
+            "block_id": collection1.key,
             "type": "collection",
             "display_name": "Collection 1",
             "description": "First Collection",
diff --git a/openedx/core/djangoapps/content/search/tests/test_documents.py b/openedx/core/djangoapps/content/search/tests/test_documents.py
index 990ec4db9dcb..47a89b2068a8 100644
--- a/openedx/core/djangoapps/content/search/tests/test_documents.py
+++ b/openedx/core/djangoapps/content/search/tests/test_documents.py
@@ -295,6 +295,7 @@ def test_collection_with_library(self):
         doc = searchable_doc_for_collection(self.collection)
         assert doc == {
             "id": self.collection.id,
+            "block_id": self.collection.key,
             "type": "collection",
             "org": "edX",
             "display_name": "Toy Collection",
@@ -325,6 +326,7 @@ def test_collection_with_no_library(self):
         doc = searchable_doc_for_collection(collection)
         assert doc == {
             "id": collection.id,
+            "block_id": collection.key,
             "type": "collection",
             "display_name": "my_collection",
             "description": "my collection description",