which is the control pin in this blower?

On a standard 4-pin case fan, look at the female connector. Locate the ridges on one side that establish a grove between them. The groove is lined up with the first through third pins, and the 4th pin is beyond the groove.

Now, standard wiring with wire color coding for 4-pin fans is:

Pin #1 Ground Black
Pin #2 +12 VDC Fixed Supply Yellow
Pin #3 Speed Pulse Signal (aka Sense) Green
Pin #4 PWM Signal (aka Control) Blue

The pulse train signal on Pin #3 (2 pulses per revolution) is generated by the fan motor and sent back to the mobo on this pin. The mobo can count it for measurement and display of fan speed, although it is not actually used for speed control. But most mobos DO also use it for failure detection - if no pulses arrive back at the mobo (or, on some mobos, if the speed is below an alarm setting) the mobo will send out an alarm. For the specific case of CPU coolers, some mobos also do more drastic action in the event of failure to prevent the possibility of catastrophic overheating of the CPU chip.

3-pin fans work differently from 4-pin fans. 3-pin ones rely on the mobo to reduce the voltage on Pin #2 for slowing down the fan. But a 4-pin fan always gets +12VDC on Pin #2. It also get the PWM (Pulse Width Modulation) signal on Pin #4. Inside the motor case is a small circuit board for several functions. One of those is to apply that PWM signal to the voltage supplied, thus changing what % of the time the current from that DC supply actually is allowed to flow through the motor windings. That is how the speed of 4-pin fans is altered. By the way, the PWM signal is usually about 20 to 25 kHz, and it is like a "square wave" except that the "% ON" value is varied to effect control at the motor.

Just FYI, this is known as the PWM method of control of the speed of these small computer DC fan motors, so they often are called "PWM fans". Now, PWM control of DC motor speeds is a well-established technology on other fields. BUT in almost all other fields, the DC motor does not carry its own special internal circuits - the actual DC voltage fed to those larger motors is already PWM-modulated by the external motor control system. You cannot use such an industrial system to supply power to a computer "PWM Fan" because it is designed differently.