A blog highlights your technical ability and showcases your ability to write blog posts. Your blog can be about different topics than those you write about for your clients. In fact, it should be on a topic that interests you. Visitors will see that you can not only write, but you can also build an online community. A good blog has the potential to earn you many referrals for more clients.
Again, if your blog has a large reader base, then businesses may be interested in paying you to review their services or products. Not only will you get a free trial using whatever these businesses are selling, but you will also get a fee for writing them a review and posting it to your site. Paid reviews (and other paid and sponsored content) can be big money, so advertise this service on your contact page to generate business.