What is part of speech to sentence dark billowing clouds?

Here's the breakdown of the part of speech for "dark billowing clouds":

* Dark: Adjective, describing the clouds.

* Billowing: Verb, in the present participle form, acting as an adjective to describe the clouds (how they are moving).

* Clouds: Noun, the subject of the sentence (if this were a complete sentence).

So, the entire phrase "dark billowing clouds" acts as a noun phrase.