I don’t see why it would be such a bad idea! I also recently just graduated and am basically in the same boat as you, so your not alone :)
I think that making a blog will somewhat make you focus more on learning new things and then writing about them in your blog for other people to read. Not only that but it can also help to keep you up to date on stuff.
In regards to using one of wordpress’ themes, I don’t think it is that bad of an idea but since your a web designer and developer, I would say try and create your own custom theme that way you can practice your design skills as well as your coding skills. It all comes in time and the more you do it, the more stuff you get better at!
Have you created a personal portfolio website for yourself yet? What you could do is create one and implement a blog in there as well so you kill two birds with one stone! It’s a good idea! :D
Let me know how it goes man!