The hardest part of blogging is hands down finding the motivation to write content. Because after you've written about 50 blog posts about a topic, you start to wonder what more there is to write, but you must continue to find things to write about, you must continue to blog, because without content, you have have no blog.
So what can one do when they run out of stuff to right about? Well, that's when you really need to get creative. I actually recommend to be always thinking about what you can write about, and as soon as you have an idea, you should put it on a list, that you can prevent being in the situation where you have no idea what you should write about.
However, should you find yourself in the situation where you have no idea what to write about. well, than that's when you need to think hard and fast. You can try meditating, or you can try reading your competitors blogs, or you can try going for a hike somewhere and rack your brain about what you should be writing about. Once you have an idea the actual writing of the blog should be relatively easy, but it's coming up with the idea in the first place that is truly the hardest part of blogging.