Sculk

Sculk is a decorative block found in the creative inventory, and created by the sculk catalyst.

Breaking
Sculk blocks can be mined with any tool, but hoes are the quickest. It always drops 1 experience if mined without silk touch.

Sculk Catalyst
Sculk blocks generate when a sculk catalyst is in range of where a mob dies, replacing any grass blocks, dirt, coarse dirt, stone, deepslate, andesite, diorite and granite in the area.

Usage
Sculk blocks are currently used as decorative blocks, but are also a good source of extra xp from mob kills, being usable in mob farms.