The BlockHarvestDrops Event is fired whenever a block is about to drop its items.
You can modify the block list as well as the overall drop chance. Set latter to 1 if you do all chance handling beforehand.

You will need to cast the event in the function header as this class:
crafttweaker.event.BlockHarvestDropsEvent
You can, of course, also import the class before and use that name then.

BlockHarvestDrops Events implement the following interfaces and are able to call all of their methods/getters/setters as well:

• IBlockEvent

The following information can be retrieved from the event:

ZenGetter	ZenSetter	Type
ZenGetter player	ZenSetter ​	Type IPlayer
ZenGetter isPlayer	ZenSetter ​	Type bool
ZenGetter silkTouch	ZenSetter ​	Type bool
ZenGetter fortuneLevel	ZenSetter ​	Type int
ZenGetter drops	ZenSetter drops	Type List<WeightedItemStack>
ZenGetter dropChance	ZenSetter dropChance	Type float

You can either addAssign the list or use the method to add an item to the list:

ZenScript
 Copy
event.drops += <minecraft:coal>;
event.addItem(<minecraft:coal>);