You only use the Black Widow key card at the end of the island, to open the sliding doors in her secret hiding place.
The key card is used late in the game to reach the final conflict with the Black Widow in her lair. That is the only place it is used.
In Counterfeit island you have to make it past the guards and get on the top level to open the doors and stop the black widow
a girl named paisley
When the Black Widow catches you , you will have to pass the guard and at top you will use your card
You use it.
Once you get the card you see the black widow on it and go to that house and go inside with the key and you will find the the painting and click on it then once you find the door use the card.
The key card does not open any house. You use it much later to enter the underground treasure trove of the black widow.
After you are in the Widow's lair, you can use it to open the final door to her trove of stolen art.
on the door inside the hideout
You get the Key Card after the scooter chase. You cannot use it until much later. Return to the museum to receive a package. In the Black Widow's lair, the guards are patrolling, but you can reach the locked door to her treasure vault (use the key card to open it).
You use the key from the Curator to unlock the Inspector's house in the CountrySide.
You find the red key card when you chase the thief on the scooter. You don't use it right away. The key card opens the door to the Black Widow's art room, where you complete the game. (You need the key from the curator of the Museum to track her down.)
Use the key card to open the treasure room and battle the Black Widow. (see related questions)
You use it late in the game to reach the Black Widow in her trove of stolen art. Its use will be immediately obvious from the card-reader on a door you encounter.