That code is stupid - don’t use it! I actually cant understand how the author got that code to work!?
The void loop() contains two delays that basically halts everything for 1,25 seconds, and all of the Blynk.virtualWrite() commands. Both are a big NO NO if you read the Blynk documentation!
Also, his approach to fetch the readings from the sensors are very awkward. There is much easier ways to do it.
Search the forum and you will probably find all the answers to your questions.
If you want to access your sensors by their unique ID, use a simple program to first identify them and write down their individual ID. Something like this:
You misunderstood me! 
roomTemperature should be declared as a float datatype because <DallasTemperature.h> returns a floating-point number i.e. one with decimals (like 27,35). When you declare it as an int you loose the decimals (27,35 will be just 27).
Good luck!