Blog Radar at VisuaLoop.
You can run a search with your favorite search engine on “visualization blogs” + “data-visualization blogs” and get about 6,500 “hits,” including duplicates. This is the weed yourself option.
Or you can choose Blog Radar and have forty (40) blogs without duplicates with three (3) posts for each one. This is the pre-weeded option.
Whether you want to expand your blog reading or are looking for a good starting point for a crawler on visualization, you will be hard pressed to find a better resource.
Enjoy!