I got it to work doing this: https://github.com/GargatheOro/GargatheOro/blob/master/TESTEVENT.java
Just add another if statement checking if the mined block is the block you want. Also, vanilla items are not in ItemInit, they are in Items. Blocks are in Blocks, which I reflected in the changed code to give you context. As far as the registry stuff, I found it overcomplicated so I put these two lines in my preInit method in the main class:
CustomEvent eventCustom = new CustomEvent();
MinecraftForge.EVENT_BUS.register(eventCustom);
The reason it doesn't use @SubscribeEvent is because the event is being registered under something else which has @SubscribeEvent.
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