It looks mostly regular.
The (classical) Latin benedictiōnem /benediktioːnẽ/. In Western Romance, changes that would apply:
- Intervocalic voiced obstruents like /d/ tend to get elided: /edi/ > /ei/
- Plosives in the coda get vocalized (when followed by another plosive): /ikti/ > /ijti/ > /iti/. Compare authority < Old French auctorité < Latin auctoritas
- Close vowels lose their syllabicity when adjacent to other vowels: /eitioː/ > /ejtjoː/
- /tj/ gets affricated to something like /tsj/, as in -tion
- Latin endings like /ẽ/ often fall off or get reduced
Combining these sound changes, we get something like /benejtsjon/. (The sound /ts/ was often written c or ç.)
The same applies for /malediktioːnẽ/ > /malejts(j)on/.
I'm not sure why the /j/ after /ts/ is lost here; it might be idiosyncratic.