Hi,
In temporary table TempBlob, FromBase64String was used to write your base64 string converted, into the Blob field.
So now if you want to write a converted base64 string into your outstream you can use the "Base64 Convert" codeunit.
There is two overloads for FromBase64.
FromBase64(YourBase64String) returns YourBase64String converted
FromBase64(YourBase64String, OutStream) writes YourBase64String converted into the Oustream passed in the second parameter.
I wrote two examples of these methods you can use for your purpose: both of these downloads a file "myfile.txt" that contains "SGVsbG8gV29ybGQh" converted from Base64 (which means "Hello World!")

=> 
Hope it will help you