"A cool wind blows."
は would more imply that cold winds tend to blow. が specifies that this cool wind is blowing を wouldn't be used here because it's not doing anything with or to anything (eg read a book, write a letter). The wind isn't blowing anything - blowing is describing the wind's state of being. That's why we use が.
は signifies the topic. It's the thing the sentence states a fact about. If the sentence used は,it could be translated as "As for the warm wind, it is blowing." In this case, however, the warm wind is part of the information we want to state, so it's marked with が as the subject of the sentence. を would mark the object, but the wind isn't blowing anything in particular, so we don't use it.
Mind you, I started this course a week ago, so I could easily be wrong.
Is it が used because literally could be translated "it's blowing a cool wind"?