Staring at a blank screen is horribly frustrating. Personally, I overcame my fear of writing by taking a class in college that required a 3-5 page essay once a week. Writing is a muscle. The more you write, the more you'll be able to write. If you need to write specs and you can't, start a journal, create a weblog, take a creative writing class, or just write a nice letter to every relative and college roommate you've blown off for the last 4 years. Anything that involves putting words down on paper will improve your spec writing skills.
So, that's why I have a blog. That's why I only feel a little bad about writing about non-work things at work. I just need to do it more.