I 've seen so many 'company' or 'corporate' blogs that are a complete mess. People were hassled into becoming corporate bloggers without having the spirit, or the energy for creating a weblog.
A corporate blog should demonstrate the beliefs and feeling of the company on current matters, serve as a news center for the company or serve as a communication tool between the company and its clients.
If you do not know what to write about, then don't write anything...