diff --git a/services/commtest/src/blob/put.rs b/services/commtest/src/blob/put.rs
--- a/services/commtest/src/blob/put.rs
+++ b/services/commtest/src/blob/put.rs
@@ -1,5 +1,7 @@
 use std::collections::HashMap;
 
+use reqwest::StatusCode;
+
 use crate::blob::blob_utils::{BlobData, BlobServiceClient};
 use crate::tools::{generate_stable_nbytes, Error};
 
@@ -7,10 +9,30 @@
 struct AssignHolderResponse {
   data_exists: bool,
 }
+
+#[derive(Debug)]
+pub enum PutResult {
+  HolderEstablished { data_exists: bool },
+  HolderAlreadyExists,
+}
+
+impl PutResult {
+  pub fn blob_was_uploaded(&self) -> bool {
+    match self {
+      Self::HolderEstablished { data_exists } => !data_exists,
+      _ => false,
+    }
+  }
+
+  pub fn holder_already_exists(&self) -> bool {
+    matches!(self, Self::HolderAlreadyExists)
+  }
+}
+
 pub async fn run(
   client: &BlobServiceClient,
   blob_data: &BlobData,
-) -> Result<bool, Error> {
+) -> Result<PutResult, Error> {
   let url = client.blob_service_url.join("/blob")?;
 
   let holder = blob_data.holder.clone();
@@ -27,11 +49,15 @@
     .send()
     .await?;
 
+  if assign_holder_response.status() == StatusCode::CONFLICT {
+    return Ok(PutResult::HolderAlreadyExists);
+  }
+
   let AssignHolderResponse { data_exists } =
     assign_holder_response.json::<_>().await?;
 
   if data_exists {
-    return Ok(data_exists);
+    return Ok(PutResult::HolderEstablished { data_exists });
   }
 
   // 2. Upload blob
@@ -57,5 +83,5 @@
     return Err(Error::HttpStatus(response.status()));
   }
 
-  Ok(data_exists)
+  Ok(PutResult::HolderEstablished { data_exists })
 }
diff --git a/services/commtest/tests/blob_integration_test.rs b/services/commtest/tests/blob_integration_test.rs
--- a/services/commtest/tests/blob_integration_test.rs
+++ b/services/commtest/tests/blob_integration_test.rs
@@ -46,8 +46,8 @@
   ];
 
   for item in &blob_data {
-    let data_exists: bool = put::run(&client, item).await?;
-    assert!(!data_exists, "test data should not exist");
+    let result = put::run(&client, item).await?;
+    assert!(result.blob_was_uploaded(), "test data should not exist");
   }
 
   for (i, blob_item) in blob_data.iter().enumerate() {
diff --git a/services/commtest/tests/blob_performance_test.rs b/services/commtest/tests/blob_performance_test.rs
--- a/services/commtest/tests/blob_performance_test.rs
+++ b/services/commtest/tests/blob_performance_test.rs
@@ -47,9 +47,8 @@
         let item_cloned = item.clone();
         let client_cloned = client.clone();
         handlers.push(tokio::spawn(async move {
-          let data_exists: bool =
-            put::run(&client_cloned, &item_cloned).await.unwrap();
-          assert!(!data_exists, "test data should not exist");
+          let result = put::run(&client_cloned, &item_cloned).await.unwrap();
+          assert!(result.blob_was_uploaded(), "test data should not exist");
         }));
       }
 
diff --git a/services/commtest/tests/identity_integration_tests.rs b/services/commtest/tests/identity_integration_tests.rs
--- a/services/commtest/tests/identity_integration_tests.rs
+++ b/services/commtest/tests/identity_integration_tests.rs
@@ -1,7 +1,14 @@
+use std::time::Duration;
+
+use bytesize::ByteSize;
+use comm_lib::tools::base64_to_base64url;
+use commtest::blob::blob_utils::{BlobData, BlobServiceClient};
 use commtest::identity::device::{
   register_user_device, DEVICE_TYPE, PLACEHOLDER_CODE_VERSION,
 };
+use commtest::identity::olm_account::generate_random_olm_key;
 use commtest::service_addr;
+use grpc_clients::identity::protos::unauth::Empty;
 use grpc_clients::identity::PlatformMetadata;
 use grpc_clients::identity::{
   get_auth_client, get_unauthenticated_client,
@@ -73,3 +80,64 @@
     "username doesn't match"
   );
 }
+
+#[tokio::test]
+async fn test_removing_blob_holders_on_logout() {
+  let blob_url = reqwest::Url::try_from(service_addr::BLOB_SERVICE_HTTP)
+    .expect("failed to parse blob service url");
+  let blob_client = BlobServiceClient::new(blob_url);
+
+  // Register user device
+  let user = register_user_device(None, None).await;
+  let mut user_identity_client = get_auth_client(
+    &service_addr::IDENTITY_GRPC.to_string(),
+    user.user_id,
+    user.device_id.clone(),
+    user.access_token,
+    PlatformMetadata::new(PLACEHOLDER_CODE_VERSION, DEVICE_TYPE),
+  )
+  .await
+  .expect("Couldn't connect to identity service");
+
+  // 1. Upload a blob with holder belonging to the device
+  let blob_data = BlobData {
+    // holder is prefixed with device ID in base64url format
+    holder: format!("{}:holder1", base64_to_base64url(&user.device_id)),
+    hash: format!("blob_{}", generate_random_olm_key()),
+    chunks_sizes: vec![ByteSize::b(100).as_u64() as usize],
+  };
+  let first_upload = commtest::blob::put::run(&blob_client, &blob_data)
+    .await
+    .expect("Failed to upload blob");
+  assert!(
+    !first_upload.holder_already_exists(),
+    "Holder should not exist yet"
+  );
+
+  // 2. Check if assigned holder exists
+  let upload_before_logout = commtest::blob::put::run(&blob_client, &blob_data)
+    .await
+    .expect("Failed to check if holder exists (before logout)");
+  assert!(
+    upload_before_logout.holder_already_exists(),
+    "Holder should exist now"
+  );
+
+  // 3. Log out device
+  user_identity_client
+    .log_out_user(Empty {})
+    .await
+    .expect("Failed to log out user");
+
+  // identity runs holder removal asynchronously so wait a bit
+  tokio::time::sleep(Duration::from_millis(500)).await;
+
+  // 4. Check if assigned holder doesn't exist now
+  let upload_after_logout = commtest::blob::put::run(&blob_client, &blob_data)
+    .await
+    .expect("Failed to check if holder exists (after logout)");
+  assert!(
+    !upload_after_logout.holder_already_exists(),
+    "Holder should be removed now"
+  );
+}
diff --git a/services/docker-compose.tests.yml b/services/docker-compose.tests.yml
--- a/services/docker-compose.tests.yml
+++ b/services/docker-compose.tests.yml
@@ -73,6 +73,7 @@
     env_file: test-commons.env
     environment:
       TUNNELBROKER_GRPC_ENDPOINT: 'http://tunnelbroker-server:50051'
+      BLOB_SERVICE_URL: 'http://blob-server:${COMM_SERVICES_PORT_BLOB}'
     build:
       args:
         - generate_keypair=true