Well I came here to ask this very question, but it turns out I'm not the only one to find their Ilford fixer suddenly dark one day.
Mine has been sitting unused for half a year (I know, so bad) in the laundry room which does experience temperature fluctuations, but it really only turned last night after an unexpected cold wave. At least that's when I first noticed it dark. It was clear a few days ago.
But I've actually been using this batch since 2010 so it's long long overdue for replacement.