I'm getting this message in GWT : Severe health issues are found in your property , Some important page is blocked by robots.txt.
Having maintained a number of wordpress sites I have found that in a great many sites the link to the admin page exists in the footer to make it easier to get to. The reason for this error is that Google has detected one or more links on your indexable pages to the wordpress admin pages and when it tries to go there it detects that it is blocked. The error is shown in case the block was an accident to give you the chance to be able to fix it and enable crawling of that page. The only options really are to...
rel=nofollow tag to all instances of the link so that Google doesn't try to crawl that page anymore
Not a big issue whichever option is taken as because all would be valid and non will cause you any issues. Furthermore having an error like this will not affect your SERP rank as Google will simply not index the page and so the page will not affect the rank of any backlinked pages.