How do I retrieve blobs (content) uploaded by a user?

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
JavaScript SDK - Content | JavaScript 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.