@samuellikesfruit The only difference between a blessing and a curse is whether it is a good thing or a bad thing. Both work the same. If you will watch when you have either spread between sims, the icon that comes up in the queue is the same. When you mouse over it, the icon says cursed.
There is a cooldown for curses to wear off for any sim you've uses a relic on so what you describe is how the game works. There is also a cool down on the relics. They can be used only so often. The way I do it is to have multiple relics when I want to complete the use 5 relics milestone. Then just use each relic on a different sim. Sometimes you can curse a sim twice but that depends on which relic is used. Most times you can't.
To remove any curse, you can go to the statue in the market area and pay to have it removed.