Consider the following scenario:
- EvilUsername registers
- EvilUsername is blocked by an administrator (autoblock is active). Special:BlockList now lists EvilUsername and #number, which is the autoblock of the (nonpublic) IP address. The latter contains the username in the summary ('Autoblocked because your IP address has been recently used by "EvilUsername".')
- An Oversight user changes the block, adds hideuser (all user contributions are now hidden, as well as log entries containing the username). The block for EvilUsername is now hidden on Special:BlockList, but the autoblock remains public. Remember, that the autoblock on the list contains EvilUsername.
This scenario is not unrealistic, it actually happens quite often, because there are much more sysops than oversight users and it's generally a good strategy to block users with evil usernames as fast as possible, before they have a chance to edit pages (vandalism is usually reverted with rollback, which means that the revert summary contains the evil username and has to be hidden manually). Expecting sysops to wait for an oversighter to perform the block is a non-solution.
There are potentially two ways of solving this:
a) Add an option to hide blocks on Special:BlockList (not prefered, as oversighters must remove the block manually)
b) Automatically hide the autoblock on Special:BlockList after a block with hideuser (prefered solution, though it may be technically difficult to implement it). I don't see any reason for implementing a) if b) was implemented.