How to use SharedAccessSignature to access blobs
Why this?
writer.WriteLine(container.GetSharedAccessSignature(new SharedAccessPolicy(), "securedblobpolicy"));and not writing the sas string you already created?
It's late and I could easily be missing something but it seems that you might not be saving the same access signature that you're using to write the file in the first place.
Also perhaps not relevant here but I believe there is a limit on the number of container-wide policies you can have. Are you uploading multiple files to the same container with this code and creating a new container sas each time?
In general I think it would be better to request a sas for an individual blob at the time you need it with a short expiry time.
Is "my.azurestorage.windows.net" just a typo? I would expect something there like "https://account.blob.core.windows.net".
Otherwise the code looks pretty similar to the code in http://blog.smarx.com/posts/shared-access-signatures-are-easy-these-days, which works.