On Mturk you upload a spreadsheet of the urls. Set up the "hits" so as to ask the worker to go the url and try to find a contact email. If they find the email they enter it in the "hit", if they find a contact form, they select contact form, if there's nothing they enter in an X or something else, you can decide. For just an email you can pay anywhere between 1-3 cents. For 300 urls you get this done in a day usually. You can put in filters for the quality of workers, I usually keep them pretty strict. You are suppose to go back and check the quality of the workers and approve or reject an answered "hit", though for the $3 - $5 that you'd be paying for this, it's not worth it, just approve all. After the turking you end up with a spreadsheet of all the urls and emails.
I'd assume you give people the blog URL and pay them to collect the e-mail address of the author. Verification could be done by having 3 different people tackle the same blog and checking that the e-mail addresses match. Pay $0.05 or $0.10 per e-mail address and you're set.