Facebook – If a friend comments on the Facebook post, will their friends see the comment if blocked

Facebook has access to all friendship-data on their site, even if you lock down the privacy settings. As an addition many users share their contact-book or even e-mails with Facebook. Most peoples have filled out their profile quite decent so it has data about your school, your employer, your current place, your interests and so on. Although Facebook states it does not use it, they can track all users (and even non-users) across many websites via the "like"-button which is transcluded directly from Facebook.

From this data it can calculate this friend requests.

My understanding of the "Hide this post" functionality is that it will hide that post and all future comments, like etc on that particular post.

If another friend posted the same picture again you would have to hide that post too. Facebook isn't intelligent enough to recognise, what are essentially, duplicate posts.