A good way to tell if blueberries have gone bad is to look at the blueberries.
When blueberries have gone bad the bad blueberries will become mushy, soft in texture and some discoloration and bruising and then mold will start to appear beginning where the stem of the blueberries was attached.
If you notice mold growth on the blueberries then they are going bad and should be thrown out.
Blueberries last between 1 to 2 weeks.
Keeping the blueberries in the refrigerator helps them to last for up to 2 weeks but it's best to eat the blueberries before 2 weeks.
If you want to keep the blueberries longer than 2 weeks you should place the blueberries in a freezer safe container or freezer bag and then freeze them.
When frozen the frozen blueberries should last for up to 6 months.