All depends on your goal:
If it is for learning purposes and it's for fun, code it from scratch. Also, try to add new functionality that you will not find in current blogging platforms. For example, make in a way that is easy for a developer to blog tutorials or screencasts.
If it's for a client or just to blog, use wordpress. You can have your site in ruby and then link to wordpress. Think about it, how many human hours are behind wordpress so for you to match that you will need to work full time on it for 8 years.
Wordpress will work out of the box and then periodically you can tweak it, depending on future needs.