Learn Italian: Le nuvole sono nere e sta per piovere. - The clouds are black and it is about to rain.

The phrase stare per plus an infinitive verb (like piovere) means an action is just about to happen. Since weather like raining doesn't have a person as a subject, we use the generic 'it' form (third-person singular), which is sta.

Could I use the verb essere and say è per piovere since it translates to 'it is about to rain'?

No. Even though English uses the verb 'to be' (it is about to), Italian requires the verb stare for this specific grammar structure. Stare per is a fixed formula for imminent actions.

Why does the word for black end in -e (nere) instead of -o?

Nuvola (cloud) is a feminine noun. Because there is more than one cloud here, it becomes plural (le nuvole). The adjective describing them (nero) has to match this gender and number, so it changes to its feminine plural form, nere.

If I wanted to say 'it was about to rain', would I change the word piovere?

No, the main action verb coming after per will always stay in its full dictionary form (the infinitive). To change when the action happens, you only change the verb stare. To say 'it was about to rain' in the past, you would say stava per piovere.