When you are in the Black Widow's secret headquarters, and go past the guards, there is a door with a lock requiring the Black Widow card. Press USE on the card and it opens the door to where you fight the Black Widow.
You have to put it in the card scanner next to the door to where black widow is :)
Enter her layer.
After you get the key card, return to the museum for the package there. You will soon have the key to the Inspector's house, and the key card is used sometime later. It opens a guarded room where the Black Widow keeps her stolen art.
The key card is used later in the game in the Black Widow's lair. Save the key card and return to the museum.
When the Black Widow catches you , you will have to pass the guard and at top you will use your card
Use the key card to open the treasure room and battle the Black Widow. (see related questions)
You do not use it when you find it. When you get free in the Black Widow's hideout, the key card will open the door to her collection of stolen treasures, where you battle her at the end of the game.
The Black Widow key card is dropped by the art thief (shady guy) when he makes his escape by boat.
When you find the scream you will be near the black widow's lair then you use it
You only use the Black Widow key card at the end of the island, to open the sliding doors in her secret hiding place.
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.
get your key and open the house that is locked and go inside