We've had an accidental situation where our Wordpress site had the robots.txt set to disallow crawling for about 7 days. I'm now trying to play clean up and Webmaster Tools is saying "Sitemap contains urls which are blocked by robots.txt." AFTER I"ve adjusted the robots.txt and allowed crawling. There is no reason why the URLS would still be blocked and when I visit the examples they appear OK.
Robots.txt URL: http://bit.ly/1u2Qlbx
Sitemap URL: http://bit.ly/1BfkSmx
My URLs where the robots.txt is blocking according to Webmaster Tools: http://bit.ly/1uLBRea OR http://bit.ly/1CsrHnr
Don't worry, if you enabled the URLs again, Google's web crawlers will notice and crawl your pages again. The message will disappear from GWT within days.
Eventually, you may test your robots.txt with GWT's corresponding feature.