1) Are you referring to case when AddOn receives different instance of IoC container? Haven't tested myself.

2) If you take a look at EPiServer.Framework.FrameworkInitialization.InitializeBlobProviders() FileBlobProvider should be registered if no default provider has been specified in configuration. If you however in 1) receive real and the only IoC container - you should be able to switch default blog factory.

But anyway - question is: do you want to swap out default provider or you want to add your provider to the list of providers?

I want to swap the default provider in an Add-on.

Regards

Then you should be good by removing current provider and adding new one: (writting without even compiling it :)

var factory = ServiceLocator.Current.GetInstance<BlobFactory>();
factory.UnregisterProvider("default");

factory.RegisterProvider({you provider here});