Why do some Blackbox challenges have a border around them?

On the home screen of Blackbox, most challenges appear as a box which is filled in with a solid color once completed. However, there are a few challenges which have an extra outline around them.

Why do some challenges have an extra outline around them?

Solution 1:

Challenges with borders are known as “meta challenges”.

According to Blackbox’s developer on the official Discord server:

Meta challenges themselves are not necessarily harder than any other challenge but they are not revealed by simply progressing through the game; they often take extra steps to find, reveal, and/or unlock.