When an Xbox 360 controller flashes continuously, it typically indicates that there is interference between the controller and the Xbox, the controller needs new batteries or there are already four controllers connected to the Xbox.
To solve the problem, one should start by restarting the Xbox. If this doesn't solve the blinking light, the next step is to check the controller's battery pack and either replace it or recharge it. If the batteries are fine, the next step is to check for any nearby electrical objects that might be interfering with the wireless connection between the controller and the console. Devices that have a history of providing interference include cordless phones, microwave ovens and wireless routers. In addition, any metal objects such as shelves or doors can interfere with the signal.