This free plugin can help you write even better copy for your blog posts as it checks your post against some typical best practices as well as keyword density and more:
Copy Compass embeds itself within your admin section for quick and easy access. Once the “Analyse” button is clicked, your copy is automatically sent through an API and is systematically and methodically analysed by the Copy Compass Core.
Once analysed, a very detailed report is generated informing you of where your copy can be improved.
Note that this isn’t an “original” concept – there are other paid services that do this as well that appear to be a bit better than this free service.