for the fuzzies thaw them in cool water or even in the fridge then give them a shot of hot water under the tap for a few seconds and then feed.. if you thaw them in warm water they go mushy.. I haven't seen one go black personally but its probably from the hot water.

Rats can keep up to a year if they are stored properly, the best way is a vacuum sealer but any airtight container will be ok for 6 months.