Abu Hurairah RA narrates that Rasulullah SAW said:
"Leave the food to cool for hot food has no blessing." (Majma'ul Awsat of Imam Tabrani RA)
The above Hadith is further supported by a Hadith graded as authentic by Imam Hibbaan RA which is as follows:
"When Saiyidatina Asma binti Abu Bakr (RA) used to cook Thareed (a special type of food), she used to cover it a while till the steam stopped. Then she used to say, 'I heard Nabi (SAW) saying, 'Verily it is greater in blessing'. " (Sahih Ibn Hibbaan)
It is quite often that people resort to blowing food and water due to impatience. This is further agitated by the hunger of an empty stomach. The hidden wisdom of observing such Sunnah is that we also learn to discipline our desires which become much weaker when we are hungry. Waiting for hot food to become cold is excellent tarbiyah to eliminate distructive animalistic behaviour when hungry.
However, the above is just a hidden spiritual benefit. We should always correct our intention that we are practicing Sunnah only to please Allah SWT.