Well, what are you interested in? A blog is a very personal (and informal) thing that revolves around your writings and the responses to you; but you are the catalyst of the discussion.
Perhaps you’re interested in international news, then look at high-caliber newspapers from your nation and those of other nations, google online news sources, watch videos.
Or if you’re more interested in science, then talk about recent advancements in all sorts of fields. You can try to cover many things or focus yourself on a certain field.
A blog has infinite possibilities, and you can talk about every single one of them, or maybe just one of them, or none of them. If you want, just comment on your daily life and see where it takes you. There’s more than 7 billion of us in the world, and at least one of them is interested in talking to you.