When blackberries go bad they will usually turn softer and mushy feeling and the color will change and deteriorate.
The blackberries will also grow mold on them which means they should be thrown out and to extend the life of blackberries never wash them until you're ready to eat them or use them.
You can keep the blackberries in the refrigerator for up to 3 days once you bought them or picked them.
After 3 to 4 days the blackberries will usually go bad to where you need to throw them out.