It is fine to give your rats blueberries, raspberries, and blackberries as long as you properly thaw them out first.
The last thing you want is for your rats to choke on anything you give them or to have trouble eating them. So far there is no documentation to prove that such food is bad for any type of rats.
Although blueberries are not a part of most rats’ diets, that doesn’t mean that you can’t get them the occasional blueberry as a treat. In fact a lot of rats like these fruits and will eat them up if you give them some.