...So, I see these "blacklisted" squares when browsing sometimes. I'm not exactly sure what purpose they serve. I assume they're images that have been blocked, or something, but why not just delete them? I can't help but be curious what they are/were.

Check under your options (in My Account → Options) if you have anything in the "Tag Blacklist" field.

If not, then perhaps those pictures have a total score (positives - negatives votes) below the value under "Post Threshold" (which is 0 by default).

The "Post Threshold" seems to be the culprit. Thanks!