Each camera works differently and the blue light might indicate a warning or an error, but since you don't have an image on the screen, then it means it's likely to be a serious error. Each CCTV camera has a button called Reset that restores the camera's settings to default state. In some cases the button is available in plain sight, otherwise you will have to open it up as protection method. Reset the camera, connect directly using a LAN camera and then check its stream. By default, the camera should display image on screen, but if it doesn't do that, then you might have to take it to servicing repair as this might be a hardware problem.