All sites on Althemy feature their own Blog section, that is enabled by default.
In your site's Blog section, you can create a new blog post by clicking on the "Add Post" button.

Here, you have the following options:
Frontpage on / off - This decides if the post will be featured on the first page of your site. This is useful when you want to keep an old structure of your page, where you already have several posts, and you don't want new posts to take their place and push them further down.
Blog Title - Here you simply enter the name of your post - what is it about.
Blog Text - Here goes the content of the post.
Add Category - When you just start with your blog, there are no categories for it, so the drop down menu (the little arrow) will give you no options. You must enter the name of the category your post relates to.
For example: A Band will put their albums into Discography category and individual bios of its members into Members or Band category.
Upload photo:
Similar to Slider and Logo, here you can chose an image to best represent your post. Click on browse and choose a photo from you computer.
The minimum size of the photo must be 600 pixels wide and 250 pixels high.
This image is the main image for your post and it is the one featured on the home page of your site if you choose to have frontpage option enabled.

You can also add various content to your blogs, like photos, videos and links.