Know Your Audience
Every blog post starts with an understanding of your audience. That goes double for choosing to write about something controversial. You don't want to completely alienate your audience; you want to, ultimately, get them on board with you regarding the controversy. Take the time to survey your audience about various topics that could be controversial to make sure you really understand them.
Ensure That the Topic Is within Your Niche
Remember the point of your blog. You can have a lot of lofty goals for your blog, but ultimately it's to attract attention in order to capture leads that will convert into sales. Your controversial post should attract visitors within your niche. If the controversy surrounds something outside of your niche, there is no real point in doing the post.
Choose a Position Based on Real Feelings
Don't try to play both sides against the middle. You won't create the controversy you want, and the post will fall flat. Choose a topic in which you can clearly take the controversial side of the argument without question. The controversial side of an argument is always the one that fewer people choose. If you really believe in the opinion and are invested in the topic, it will help you move forward with your blog post.
Back Yourself Up with Facts
Try to find a lot of statistics to help back up your opinion. As someone once said, "You always have a right to your own opinion, but you don't have a right to your own facts." Using facts to back up your stance will go far in making your post readable, understandable, and logical, while still keeping the controversy that will propel your blog post to become viral.
Be Willing to Change Your Mind
When presenting your argument, invite others to try to change your mind with facts. This is a great way to lessen the blow of your alternative opinion about a topic within your niche. If you're not being one sided, but leaving some sides open for exploration in comments, you can keep the controversy going longer in the discussion portion of your blog post.
To recap, remember to keep the topic within your niche, take the minority view (but only if you really believe it), back yourself up with facts and statistics, show examples and allow an open dialogue to ensue after your blog post. Be willing to change your mind once there is discussion based on sound reasoning and facts.