When the Black Widow catches you , you will have to pass the guard and at top you will use your card
The key card is used later in the game in the Black Widow's lair. Save the key card and return to the museum.
You do this near the end of the game, when you are in the lair of the Black Widow.
The Black Widow key card is dropped by the art thief (shady guy) when he makes his escape by boat.
You only use the Black Widow key card at the end of the island, to open the sliding doors in her secret hiding place.
It will.
you get in the boat and sink
skrow you
The key card does not open any house. You use it much later to enter the underground treasure trove of the black widow.
The key card is for later. Return to the museum, where you have a package waiting for you.
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.
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.
Use the key card to open the treasure room and battle the Black Widow. (see related questions)