Is it possible to directly access the URL of a public image using blob id?

Yes, it is possible. You can upload a blob and make it public. For more information, please visit: Content.

Or see relevant information in SDK documentation:

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.