I however, blog and post alot about scientific topics on various message boards. Can I put this on my resume as writing "experience"?
I would have to guess that, sadly, unless you're blogging at a level sufficient to be famous for it (at least among members of the profession for which you hope to write), then blogging and message board-posting won't do much for your resume. Unless the message boards/blogs are famously discriminating in their choice of whom to admit and whom to reject, then the targets of your resume have no way to determine whether you're posting gems or junk, and they probably won't bother visiting the site to find out for themselves.
I'd have to agree that good writers who are scientifically literate are in short supply. While in college, some of the most amazingly bad stuff I read was written by men and women far smarter than I am but who were unable to pen a coherent sentence to save their respective lives.
Perhaps a place to start is to submit articles to recognized science journals or academic publications. You won't make a ton of money outright, but every article published in such a forum is another addition to your portfolio...