The verb is "was", past tense of "to be". All sentences have to have a verb.

"...the sky a perfect, cloudless blue" is perfectly correct. First, it's what we call an elliptical sentence, so our understanding of it depends on the first part of the sentence, "It was seventy-five degrees in Phoenix..." Secondly, the action hasn't changed, so repeating the verb "was" is redundant. Thirdly... read it aloud! You will hear this part has a perfect weak-strong rhythm. So, there's a poetic element.

- "It was seventy-five degrees in Phoenix." The verb is 'was'. the sky a perfect, cloudless blue" This part of the sentence is totally wrong. Correct would be: It was seventy-five degrees in Phoenix, the sky was perfectly clear and blue. or: It was seventy-five degrees in Phoenix and there was a perfectly clear blue sky.