An essay is exploratory. It starts with an idea, handles it, studies it, and looks for a meaning to come from it. Although we usually think of an essay as simply writing down an idea we know and supporting or defending it, that is merely a side-effect of planning and drafting.
Think about the writing prompts you get. Whether it be simple ones like, "Which month is your favorite and why", or complex ones like, "How does the main character's relationship with his father affect his relationship with his son", they require equal amounts of thinking.
You can easily brush it off and write something plain and banal, or you can seriously ponder these questions, self-reflect, and discover the answer. In the latter case, your essay is more discovering your findings than it is being a sterile collection of simple answers.
What is a blog? It's a person's thoughts, recorded. Some may be simple observations ("I like it when it rains."), and some may be deeper introspections ("I like it when it rains because. . ."). And a good blog post is a good essay. You will be doing all the same thinking and exploring you would. The only real difference is that we assume one to be written on paper and the other written on the infinite canvas.
If you want to write, but aren't sure of what, or if you are lacking the confidence to put it on paper, start a blog. There is less pressure (despite it being on the internet, we feel like nobody is watching), and more freedom. You can make a post be a paragraph or a page (or more if you're feeling adventurous). And the writing you do for a blog is plain ol' writing. If you can do a blog, you can do any form of writing. Use it as a first step. Then proceed from there.