When a blocks is operated by a user A until date D, if user B re-blocks the user undefinitely on date D-1, it is user A who will appear as the blocker in the block list (Special:BlockList), even after date D.
Example: cf. undefinite block of Delarouvraie operated on July, 22 2018 on fr.wiktionary (https://fr.wiktionary.org/wiki/Special:BlockList), operated by Pamputt (cf. https://fr.wiktionary.org/wiki/Sp%C3%A9cial:Journal?type=block&page=Delarouvraie)