led blinking through serial port

We do not do your homework: it is set for a reason. It is there so that you think about what you have been told, and try to understand it. It is also there so that your tutor can identify areas where you are weak, and focus more attention on remedial action.
 
Try it yourself, you may find it is not as difficult as you think!
 
However, I would suggest that you look at connecting it to one of the control signals that you can control (look at the SerialPort class[^] at look at RTS and DTR) Use a low power LED - you can damage the driver chips in the sreial port if you try to pull too much power.