Site is down for me at the moment so I'm not sure I understand what you mean by "hidden blank field" but I have a niche social network with around 150k pageview

Site is down for me at the moment so I'm not sure I understand what you mean by "hidden blank field" but I have a niche social network with around 150k pageviews/month and a simple css hidden input field on the signup form has been enough to keep all the spam away.