I want to blacklist only the main site - Stayfocusd was a particular example, I got some google results on subreddits that I never knew existed and couldn't have been in a whitelist.

I'm rather going for the reading vs scanning distinction - even there I would want the particular subreddit main page to remain blocked to prevent the procrastinating pattern of checking an interesting place to see if there's something new interesting. Similarly, the main news.ycombinator.com should be blocked but visiting particular pages from e.g. a weekly digest email would be useful.