A lot of businesses want to start a blog, but they're not sure what exactly they should be posting there. Company updates? Larger news items that might be of interest to their clients? Economic developments affecting their industry? General "Reader's Digest"-style news and jokes? Recipes?
The answer is yes to all of these, except maybe recipes. Whatever your clients want to be informed about - or even more to the point, whatever they want to read day after day - is what should be on your blog. If you ran a tax consultancy, that might be changes to the tax code or new strategies you've been hearing about. If you're an advertising firm, it might be about a new designer you've hired, with links to some exciting new graphic looks. If you run a scrapbooking site, it might be homey personal-interest stories.
Every blog should consist of what people want to read. And obviously, that will be different for every company. There are ways to monitor your page views to see what prospects are clicking on and what they're ignoring, to see if the content you're posting is working. And if it's not, we'll keep trying till we find the right formula to keep people coming back.