Following T189073: Refactor logic for creating and logging a block out of SpecialBlock so it can be easily reused elsewhere, we should update SpecialInvestigateBlock to use the new BlockUser service, instead of the deprecated method SpecialBlock::processForm.
As well as improving the architecture, this will fix an error I encountered locally (unlikely in real life), when a user tries to block themselves - filed as T257918:
ErrorException from line 789 of /var/www/html/mediawiki/core/includes/specials/SpecialBlock.php PHP Notice: Undefined index: PreviousTarget
This is due to SpecialBlock::processForm not expecting to be called from external forms that don't necessarily have the same fields as SpecialBlock.