The problem with a blog is the organization. Many times it is date and category based, and you can better sort a wiki. Also I have found the speed in editing and posting to the wiki to be faster and simpler.

Another issue unless your running your own blog is most blog services run public, and you probably just want to have the discussion with your co founders and advisers. Most blogs also don't provide revision support which we found to be very useful when multiple people are editing and working on a file, a good safety net.

Blogs are great for publishing and archiving those items, but not necessarily the best for continuous brainstorming.