At present, blockchain is known not only as decentralized but also as cryptographically protected. So to say that it's simply storage of data or just front-end would be wrong today. Still, let's look at the main difference between these two terms to understand this technology better.
Back-end (or back office) is a term that was first introduced in software development. This term can be defined as a set of programs and servers on which they are running, which support the primary system. Thus, the back-end is not visible to users; it simply provides all necessary functions.
The front-end (or front office) is what users see when they use an application. If the blockchain is used in the project, it will be visible to users (if not, they can hardly understand what this technology is about). IBM developers introduced this term in the context of programming languages like HTML and JavaScript. So this part of the application should be attractive enough to draw users' attention to your product.