There is no basic way to retrieve blobs uploaded by a user if files are private.
You can download only public content:
iOS SDK - Content | iOS SDK | QuickBlox
Android SDK - Content | Android SDK | QuickBlox
React Native SDK - Content | React Native SDK | QuickBlox
Flutter SDK- Content | Flutter SDK | QuickBlox
Feel free to check our official documentation for more information that will help you to integrate QuickBlox services into your applications / projects or contact our Support team by submitting a ticket if you have any other questions.