Developer B died for the first time on me when I used to just store it in it's original bottle. I then switched to putting it in an amber glass bottle and storing in it the fridge. It still died as did unopened bottles. If you follow the discussion in the second link there's some stuff on how long the developer ought to last and how to read the code to find the manufacturing date.

David Lyga, here, says that it's fine to use the black developer B. I've never tried as I only get into the darkroom very occasionally at the moment and I've got no time to experiment. What I now do, though, is to mix up all 5 litres in one go and store them in 1 litre pop bottles. A technique I got from APUG member hrst and I've printed with 18 month old developer with no issues.