There are many ways to grade blogs, and what you choose to do depends on your aims. Here are a couple of tips form a ProfHacker post.
Both Jeff and Julie look for thoughtful responses, good writing, original ideas, taking advantage of the medium (linking, video, audio) where appropriate, and, of course, actually posting the blogs on time. [Jeff notes that assessing the blog-based projects is a little more complicated.] For Julie, at least, her general rubric for blog evaluation is a slightly modified version of Mark Sample’s rubric in “Pedagogy and the Class Blog”, which she suggests everyone read at some point when working through this question of evaluation.
For Jeff, since in some cases he asks students to post twice a week, in at least one class he has done two grades for blogging: one for an overall grade for all blogging, and a second grade, in which students picked their two best posts, revised them, and then submitted them for a separate grade. On the other hand, Jeff says he has a colleague who only spot checks to make sure that students have completed the blogs, but then has them create a portfolio of the best 2-4 posts at the end of the semester for him to provide a grade. Because Julie has been fortunate to have small(ish) classes (typically less than 30 students in each), she reads and assigns a grade to each post, and for the first several weeks offers a comment on each until the communities run themselves. (Source)