I believe that blogging is an important 21st Century skill because of all the area it encompasses. Not only is it 21st Century Writing, but it is also Reading. It is also much more.......
It is Communication ~ in a digital form.
It is Collaboration ~ in that student can begin to dialogue and discuss using a platform that allows all students an equal voice, even those with a quiet voice.
It is Critical or Higher Level Thinking ~ prompts that students are given are crafted to stretch their thinking and allow them to express their opinions and viewpoints with supports and evidence.
It is Creative ~ in that students have thinking time to develop creative responses and this allows for more quality in their contributions.
It is Reflective ~ students are given prompts that allow them to reflect and respond on their own learning while making connections with new information and their own world.
Here are a few sources to get you thinking about what general, creative, thinking (not content specific) blog prompts (or posts) could look like.
Marzano Question Stems ~ being stems, these could be aligned well to your specific curriculum
The Daily Post: Daily Prompts
The Daily Post: 365 Days of Prompts ~ these two sources have a great variety of creative, thinking prompts. Again, not content specific , but would inspire great thinking
My Pinterest Picture Writing Prompts
Visual Writing Prompts ~ I love the visuals that help inspire student' thinking and creativity
Video Writing Prompts ~ very cool and lots of fun
Writing Forward Writing Prompts ~ again different categories of thinking prompts