What have you blocked in robots.txt? Often GWT complains about files being blocked that you have purposely blocked so if you have a directory that is off limits, the blocked files may be in there. Or you may have blocked a file extension like .css or .gif and those are files that you intended to block. Not all the notifications in GWT are very useful and like lucy24 I have not found where they list blocked URLs so that a person could verify that they do or don't intend to block them.
They complain to me about getting 403s and the URL shown is one that is clearly blocked in robots.txt because they are not permitted to crawl that type of URL. Now, how did they get a 403 if they are reading and complying with robots.txt? Beats me.