In that case, Google Webmaster tools is not actually reporting an error. That's a report to show you what URLs Google tried to crawl but couldn't (due to being blocked) so you can review it and ensure that you are not accidentally blocking URLs that you want to have indexed.

I agree that it's confusing in that the report is in the "crawl errors" section.

(I built Google webmaster tools so this confusion is entirely my fault; but I don't work at Google anymore so sadly I can't fix this.)