Duskberry Bushes are naturally occurring blocks in the Nether that spawn Duskberries.
The Duskberry Bush is an easily transportable block. They infinitely spawn Duskberries. You need to tap the bush in order for it to drop the Duskberry. The Duskberries can be easily seen on the full-size (grown) bushes, as they appear as black dots. Only harvest when you see the Duskberries on it.
When transplanted, they will need to grow to their full form to spawn Duskberries. They need to be placed on netherrack.
You can use bonemeal on the bush to make it grow through the stages faster as well as produce another bush on top once that bush is fully grown.