Can I develop Blackberry applications using C# .NET on windows7

No. You can't do it. You have got to use Java to do Blackberry development. There use to be a C/C++ API for blackberry but it is no longer supported.

Check this stackoverflow question out.

Supposing you talk about BlackBerry Plug-in for Microsoft Visual Studio 1.2:

The BlackBerry® Plug-in for Microsoft® Visual Studio® allows web developers to leverage existing Microsoft® based developer tools to create and debug web projects for BlackBerry smartphones.

• see video lesson
• read Debugging Your First Web Application In Microsoft Visual Studio
• look into VS plugin docs and related KB articles