It sounds like you want casting, this youtube video helped me understand exactly what it is and most importantly what the object node is for and why it’s important. - YouTube
With casting you’ll be able to get and set or otherwise perform actions as what ever blueprint you’re casting to from inside a different blueprint of the same class.
So when your ball is destroyed you would want to cast to what ever blueprint you’re storing that variable in, from there drag from the “as (name of your blueprint)” pin and you can get or set the variable you want.
Assuming you have the right object pin and the cast succeeds that should do what you want.
PS: For casting from a class blueprint to a level blueprint that requires Event Dispatchers if I’m not mistaken. This video might help you with that one. I haven’t experimented with them yet so best of luck to you. UE4 Event Dispatchers - YouTube