I am pre-heating my tank but I realize plastic probably doesn't hold the heat well...I suppose I was counting on 3:15 (or 3:30) as not being that long for the heat to need to hold.

I'm feeling a bit like I over-panicked. Perhaps it was the blix after all, because I just scanned some of the new negs from last night (after I mixed blix correctly instead of trying my separated version) and the problem is not nearly as pronounced as before.