If you want to use a simple button, then you need to use virtual pins, a servo library and some basic code.
https://www.arduino.cc/en/Reference/Servo
http://docs.blynk.cc/#blynk-firmware-virtual-pins-control-blynk_writevpin
https://examples.blynk.cc/?board=ESP8266&shield=ESP8266%20WiFi&example=GettingStarted%2FVirtualPinRead (Use Button Widget instead of slider).
BLYNK_WRITE(V0) // This function gets called everytime the Virtual Button on V0 changes state
{
int value = param.asInt(); // Get value of button as integer
// Place servo control code here based on value data.
}