I believe that saying one coherent thing in a blog post means leaving or cutting out most of the detail. I picture it like this – everything I write is like a map at some level of zoom. If we are zoomed all the way into the city of Denton, Texas and describe everything in it then we will not be making a coherent point. What we write will be mostly boring. So we would zoom out a little on the map and just describe the essential things to make our point to describe the essence of Denton.
But if we zoom into a detail about Denton – maybe the animal shelter – then there is a whole new coherent story to be told. At that level of detail we have to leave out the rest of the information about the city. We can zoom closer still into the story of one of the dogs at the animal shelter and still tell a coherent story. The amount of detail corresponds to the size of the thing we are describing.
This follows onto my point a couple of days ago about finding new topics in old ones. Adjust the zoom a little and you have an entirely new story to tell.