Lower Voltage of 15 VDC Power Supply to 13 VDC for DC Blower
Will 15 V hurt a 12 V blower?

Maybe.  What does the datasheet say about absolute maximum and operating maximum voltage?

Reducing the voltage at this power level is the wrong way to go about this.  Get the right power supply.  12 V is a common power supply voltage, so this will be easy.