WHY THE BLUE-BLACK SMOKE WHEN COLD STARTING?

Oil or, if white smoke also appears, its from coolant,. Typically really dark smoke signifies that just the valve seals alone need replacing. you will have oil slipping past the valve stem seals that have atrophied and shrunk with heat and age. Valve seals are a cheap fix that will definitely prevent premature spark plug failure and improve combustion.

Lighter smoke usually indicates further problems as stated in:

HOW TO IMPROVE START UP, WARM UP, IDLING AND ACCELERATION