The blog is a co-constructed online space that rivals the website as a publication platform. While adults, teens and children author blogs, little is known about the younger participants’ practices. Further, despite the blog’s interactive nature, the impact of collaborative affordances on meaning is yet to be fully articulated. This paper reveals how blog authors employ the technical affordances of blogs to position readers as meaning co-constructors. Child blog authors and their readers achieve three types of co-authorship by using tags and comments, as revealed by a systemic functional linguistics (SFL) analysis. Analysis shows that blog co-authorship blurs the line between ‘author’ and ‘reader’, unsettling traditional notions of writing and reading. Curriculum and policy definitions of literacy that separate reading and writing are placed under pressure by the co-constructive nature of web-mediated texts of power such as blogs, suggesting that collaborative text realisation may need to be accounted for in future curriculum design. © 2019, Australian Literacy Educator's Association.