Page 3 of Unseen Forces rules wrote:
After each roll (and after all rerolls, abilities, or terror effects have been resolved), if the investigator’s black die result matches the result of any other die in his dice pool, he discards the black die and the matching die from his dice pool. If more than one die matches the black die result, the investigator chooses one matching die to discard with the black die. The highlighted bits pretty much cover it.
The black die stays in the investigator’s dice pool until it matches the result of another die and is discarded, or the investigator ends his turn. The black die cannot be discarded as the result of a failed roll, does not trigger terror effects, and cannot be affected by item and spell cards, investigator abilities, or clue tokens. For example, it cannot be secured by a Spell card, and its result cannot be changed by Marie Lambeau’s ability.
You add it to your Dice Pool and roll. If you get a match the Cursed Die and matched Die are Discarded from your Dice Pool and remain that way until you next build your Dice Pool (usually on your next turn).