Code

#define BLYNK_PRINT Serial

#include <ESP8266WiFi.h>
#include <BlynkSimpleEsp8266.h>
BlynkTimer timer;

char auth[] = "*******************"; //here goes your Blynk authentication code

char ssid[] = "ABC";  //here goes your username
char pass[] = "******"; //here goes your password
int flag = 0;

void notifyOnButtonPress()
{
  int isButtonPressed = digitalRead(D1);
  if(isButtonPressed == 1 && flag == 0){
    Serial.println("Someone Opened the Door");
    Blynk.notify("ALERT: Someone Opened the Door"); 
  }
  else if(isButtonPressed == 0)
    flag = 0; 
}

void setup()
{
  Serial.begin(9600);
  Blynk.begin(auth, ssid, pass);
  pinMode(D1, INPUT_PULLUP);
  timer.setInterval(16000L, notifyOnButtonPress); 
}
void loop()
{
  Blynk.run();
  timer.run();
}