I believe when Lupton speaks about linearity she is talking about the flow of information. For example when in conversation you can’t fast forward and get to a certain point of the conversation because you have no idea what another person is going to say, and even if you did it’s just impossible. You simply have to listen to the information as it’s given to you. However when information is presented to you in a textual form you have more liberty with how you read it. You can skip around and only read certain sections or skip everything until you get to the main point. And tools that come with a text, such as the index, appendix, abstract, footnote, and table of contents, help direct you to different parts of the text and therefore you are not gaining the information in the same linear form that it is presented.
I’d like to think that a blog is a linear form but ultimately thats up to the blogger. For most blogs, when you post, your posts are presented in the order that you’ve posted them whether it be from latest to oldest or vice versa; there is a linear quality to them. However if someone is a more advanced blogger they might set up their blog a bit differently. Instead of having a linear flow of information you could break up your posts into different categories on different pages. So you separate all of the information and no longer have this linear flow of content. It all depends on what the blog owner wants. Regardless of this however, even if the blog was in linear form there are also tools that may be available to navigate you away from the linearity of it.