@ErfanDL There’s a problem with your logic for the if statements. There are gaps where there are no criteria to fit certain readings. For example, values between 251 and 300 and a value of 400 have no matching if criteria.
Personally, I prefer to use a Switch Case statement with ranges rather than if statements. Here’s a re-work of @Toro_Blanco’s code to use Switch Case (and fixing the gaps in your criteria).
int airQuality = analogRead(A0);
case 1 ... 250:
case 251 ... 300:
case 351 ... 400:
case 401 ... 999:
The default option at the end will print the analogue read value to Blynk if its outside the range of 1 to 999. Not sure if it’s needed, but could help to debug dodgy readings.