-Avraham was placed in a dungeon before he was taken out to be burnt.
-Nimrod made sure the dungeon was pitch black, after an extended period of time he took him out into bright sunlight.
-Avraham’s many merits saved him.-Meaning Nimrod knew that blindness would limit Avraham's ability to perform a miracle 
(Medrish Talpiyot, branch of Avraham.)
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