Blob URLs are stored in the tblContentLanguage table. (BlobUri and ThumbnailUri columns).
Use Microsoft Azure Storage Explorer or ask Optimizely support to verify that blobs are copied to the correct location.
Hope this helps!
If it is any help this plugin may help with download missing blob files when restoring a site locally.
If you have access to the PaasPortal you can export the databases.
You could also request that Optimizely provide you with a backup of the database and blobs and place them on the integration storage account for you to access.
Is it possible to capture the request using something like fiddler and inspecting for invalid characters?