I wouldn't. Sorry to be a buzz kill, but if I wanted to sharpen my skills, I would either find an open source project to help out on (maybe even a blogging one) or work on a side project that either helped people or had a chance of making money.
Building a blog engine isn't breaking new ground and I might as well add features to an existing engine, where I won't have to build everything from scratch, and I can add/build any features I feel that I need.