Blogs help with this in at least two ways.
First, blogging is a way of exercising the freedom of speech. The simple act of speaking out increases the amount of democracy in a country because freedom of speech is such a central aspect of democracy.
Second, blogging can help to bring about other aspects of democracy. Blogs can reveal to other people in a society that there is a great deal of desire for democracy. If you are in a relatively authoritarian society, you might not know that other people wish to have democracy. Blogs can tell you that such people exist. If more and more people come to realize this, they can form a serious movement to promote democracy. This can, ideally, help to bring about more democracy in a society.