The mass block CheckUser interface should be split to a separate special page.
We could still display the form on the CheckUser results page, however, it would POST to a different special page.
The special page could then be enhanced to support further blocking options like T24120: Enhance Blocking and tagging from checkuser interface: add dropdowns.
Also, separating it from CU permissions would enable us to open it up to all administrators (and possibly move it out of CU?).