Take a cotton swab and some medical isopropyl alcohol. If you have the option not to dip the cotton swab into the container, don’t do that, but I have still had decent results with doing so after drying a touch. Run the q-tip along the contacts of the game you are having the blue screen error with and you should see the tip turn golden or black because these carts had 30+ years to accumulate dirt. Finish cleaning both sides and dry with anything but your breath or you can just choose to oxidize the cart. It’s your choice, your game.
If option ^1 doesn’t do it for you, I’d go find a long Philips 1 screwdriver and prepare to spend 20min-1hr opening up your console so that you may depress the contacts on your NES. This can be achieved by using a thin yet sturdy fork to pull the contacts in your loader pins out towards you with as little malformation to the pins as possible. You may use the alcohol soaked q-tip method here as well in case your pins received a thorough enough dirt bath from your games.
These methods fixed every problem I’ve had and I’ve gone through all of the color codes. If you have any more questions, I would appreciate if we create a running dialogue for any others reading so that this ends up being the last post on the site about the blue error code. Have a good day and I wish you luck with your almost new plastic box.