Is there a way (without using third-party module) to let visitors of a portal recommend articles of a blog to other through e-mail?

I see your need, but unfortunately, DotNetNuke currently does not provide a concept of "deep  linking", i.e. to provide this feature for a single blog post, this would be necessarily implemented by the individual module. Currently this is not supported by the blog core module, and in fact, i would prefer a general approach in the core framework instead.