I have often frozen blues after picking. If you insist on washing before freezing, you need to spread them out to completely dry before freezing. Most people don’t wash them.
You can freeze them in a couple of ways. Some people spread the berries out on trays and pop in the freeze to freeze before bagging. Others put in a bag and stick them in the freezer.
I have had success with both methods When ready to use, all you need to do is take the bag out of the freezer, pour out what you want and pop the bag back in the freezer. (If the berries are at all wet when you freeze them, they will stick together when they freeze and you will have problems getting just what you want out of the bag.) If you only want them for your cereal, you might want to try dehydrating them. Hope this helps, Mary