When you are evaluating blogs for their teaching and learning potential, you have to consider general aspects of teaching and learning, as well as some specific things which are related to the online experience. Start by thinking about what you would consider to be useful / helpful / essential / desirable in a blog.
You may get some ideas from this page: http://thedigitalwriter.blogspot.co.uk/p/criteria-for-evaluating-research-blogs.html
This page from the University of Education MSc Digital Education is not specifically about evaluating language learning blogs, but you will find the concepts and focus interesting: http://onlineteachingmanifesto.wordpress.com/
As part of an evaluation, you may want to set up your criteria, as a kind of check list. To pilot your checklist, why not ask some friends / colleagues to look at your own blog and then to complete an evaluation. Here is an example (use the “add poll” link on the toolbar for editing a blog):
Note – this is a very simple poll – I’m sure you can create a more sophisticated one!
One important contribution to thinking about online learning (not just blogs) is given by Reuben Puentedura and is known as the SAMR model. You can find out more on his blog http://www.hippasus.com/rrpweblog/, and it would be useful to consider the SAMR approach (we will return to this later in the course).