The worst thing that would happen if you eat bad blueberries is that you get an upset stomach, diarrhea, stomach cramps etc.
You might feel nausea and then after a day or so you should be feeling better.
You're unlikely to get deathly ill or seriously ill when eating bad blueberries.
But if you have old bad blueberries you should throw them out though and not risk getting sick from it.
If the blueberries smell bad, have mold on them etc then throw them out.
Blueberries should last when frozen for up to a year and in the refrigerator the blueberries should last for a week or two weeks.
Out on the counter the blueberries would usually last for a week or two as well.