Unless you're a front end developer, what the end product looks like is of little consequence. If you're a back end developer doing mainly closed-source applications, a portfolio is not a good way to showcase your work, since by definition there will be little or nothing you're legally allowed to showcase.

Instead, focus on other ways to show what you've done and - more importantly - what you know. Blog posts are easy but it's unlikely prospective clients will dig through blog archives to find a problem relevant to them. Instead, you could likely discuss an application in vivid technical detail without mentioning the client and not be breaking any NDA or contract rules. Think of it as a white paper to host on your site as something a little more in-depth than a blog post.