I will answer two major questions for you and point you in the right direction
How to save articles for blog? Whether its good idea to use XML for saving article's data?
You can wither put them in a database or XML file but which one you choose depends on scale. If you need to serve a lot of visitors then perhaps database like sqlserver is the best option. If ou want to run it in intranet then XML could also be a good choice.
Now you can find this blogengine useful to understand the various ASP.NET, degign and n-tier architecture related issues.
YaBlogEngine - A Tiny Blog Engine written in ASP.NET/C#[
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