QR codes are those pixelated squares appearing on everything. They are essentially like bar codes and contain information, usually a link to a website. There are plenty of free QR code generators available online and they can be read by bar code reader apps available for most smartphones.
QR codes can be generated for each blog post a child makes and displayed next to them. If pupils have work in exercise books that relates to their blog posts, they can stick a copy of the QR code in their book to keep a link to their work and any comments it receives in one place.