Why does my NES game only show a blue screen?

Well, this is the most common problem with the NES. Now, you know how when you slide the game in the system and then push it down? Well, the reason you have to push it down is for the game to come in contact with the game pin connector that reads and plays your game. What happened inside your NES is that the pin connector has worn out,since it has been used out. What you need is to do is go on any online store(your preference) and purchase a new 72 pin connector. After getting it, you must open your NES up and replace the connector. Now, do this when you have plenty of time cause it is very tedious and time consuming. First, flip the NES over. There should be 6 screws. Unscrew them with a screwdriver(duh) and flip the system back over. The top should come off. Now you should see a black cartridge spring loading thing, or whatever, and unscrew it, and slide it out. Don't try to pull it up or it will break. Now, you should see a black rectange with little golden pins on the mother board. Slide it off the board. It should take a little force. Remember, this thing is about 20 years old and has been there for a while. With your new connector, slide it on. Repeat the steps backwards and assemble your pitiful NES. If you did this right, all of your game should work like they did 20 years ago! If it still dosen't play the games, get some windex or lysol or some other generic cleaner, some Q tips and just run it back and forth on the cartridge board. Enjoy your like-new nintendo!

I have had the same problem, I heard replacing the 72 pin with a new one will make the blue screen disappear. But when it blinks that mean the console thinks that you're playing an illegal game and to get rid of that you have to dispose of the 4th pin (the lockout pin) and that will cure your problem.