I have some very definite opinions on this, relating purely to people who are seeking journalism jobs. So here they are.
Yes, you should blog. Don’t write about your freaking cat or where you got drunk last night. Also, don’t blog long essays on your personal opinions about the world. (Unless your essays are of the same quality as Leonard Pitts’. Which is not terribly likely. Sorry — but hey, mine aren’t that quality either.)
Here is what I suggest, because this worked for me: Blog news-like items about stuff that interests you. This is the equivalent of giving yourself your own “story assignments” — which is a joy beyond measure. Write things that are useful, interesting, clear, stylish; illustrate them with photos, videos and hyperlinks; show that you respect proper protocols about using images or materials from other sources. I’d say you don’t have to nail yourself to AP style, but be clear, clean and well-spelled; never write anything libelous or sketchy; use the blog as an excuse to try out bits of online media gadgetry like Twitter, Instagram, Dipity, whatever’s hot.
This says so much about you. If you are normally a photographer or a copy editor, this gives you a chance to stretch your writing skills. If you’ve been an assigning editor for years and don’t have any recent clips, this can be a warmup exercise.
Your employer might require that you give them first refusal on anything you write. Great — do it. If they run it, you’ve got a published clip! If they don’t, you still have a good blog post.
Are blog posts legitimate examples of your writing that you can show to a prospective employer? Sure — as long as it’s clear what they are. They are obviously not pieces of work assigned or accepted by a major publication. But if writing is good, it shows. If writing rambles on and on with crappy grammar, irresponsible snarky comments and no real point, that shows, too.
And, ever so possibly, writing about your greatest interests gives you an excuse to connect with people in that field or hobby, which could benefit you in ways we can’t foresee down the road. I have a friend who’s very knowledgeable about green building, and I so much wish that she would start blogging about it, because that is such a fast-growing, nationwide, fanatical audience. You never know.