Never mind what I said about the adverb. It could have been a sentence about the clouds “easily obscuring the moon,” and it would have been a noun phrase… It had nothing to do with the adverb, and everything to do with the kind of phrase the phrase was.
Here are some rules:
A. Coordinating and subordinating conjunctions can connect clauses together. If the two clauses share a subject, or share a subject and verb, the coordinating conjunction can omit them in the second clause (forming a predicate phrase or a noun phrase); these phrases must agree with the subject or subject and verb of the main clause. The subordinating conjunction cannot.
(1cc) The dog ate his food, and the cat played with her felt toy.
(1sc) The dog ate his food, although the cat played with her felt toy.
(2cc) The slinky slunk down the steps, and [it] stopped once it reached the bottom
(2sc) The slinky slunk down the steps, although it stopped once it reached the bottom.
(2cc) Katie heard a bird singing outside her window, and [she heard] a siren wailing in the distance.
(2sc) Katie heard a bird singing outside her window, although she heard a siren wailing in the distance.
B. Coordinating and subordinating conjunctions can connect adjective phrases (2), and adverb phrases (3) to a clause. These phrases need to match with the subject and verb of the sentence, as if you could insert the subject and verb into phrase and make a separate clause.
(2cc) The events that took place today were necessary, but [they were] regrettable.
(2sc) The events that took place today were necessary, although [they were] regrettable.
(3cc) They fought the war valiantly, but vainly.
(3sc) The fought the war valiantly, although vainly.
C. Coordinating conjunctions and prepositions can connect noun phrases to a clause.
(4cc) Katie heard a bird singing outside her window, and a siren wailing in the distance. [Yes, same example as above]
(4p) Katie heard a bird singing outside her window, despite a siren wailing in the distance.
D. Phrases with subordinating conjunctions and prepositions can precede the main clause. (Coordinating conjunctions are always on the subsequent clause(s), even if that clause it its own sentence).
(5sc) Although the Stepfords were often late to parties, they always brought delicious cake.
(5p) Despite often arriving late to parties, the Stepfords always brought delicious cake.
[Notice the difference in the initial part of each sentence—5sc has a SC and a clause; 5p has a P and a noun phrase.]
Now it is a matter of correctly identifying which rules to use.
We have “_______ easily obscured by cloud cover, blood moons are fairly common.” We have an incomplete phrase followed by a clause. The phrase needs something to precede it, and with D we can see that either SC or P could do so. Now we look at the phrase. What part of speech is “easily obscured by cloud cover”? It’s not a clause—it lacks a subject and a verb—so we can ignore A for now, since the only option for SC in A is a clause. So is it an adjective phrase, adverb phrase, or noun phrase?
For that, we have to find the main word of the phrase—the part of the phrase that is carrying the bulk of the meaning. In this case, “easily” and “by cloud cover” are both modifying the word “obscured,” which is not modifying anything in the phrase.
On its own, “obscured” could be the past-tense of “to obscure,” or the past participle. For it to be the past-tense, the subject of the sentence would have to be the entity which obscures (the clouds), but in this case we have the moon, the thing being obscured. The moon is obscured—“obscured” is acting as description—so the phrase is an adjective phrase. (Additionally, if “obscured” were the past-tense of a verb, this would be a predicate phrase, necessitating a CC (see A), and violating D).
We look up at our rules, and we see that by B, CC’s and SC’s can connect adjective phrases to the main clause. However, from D, we already know it can’t be a CC, since the phrase is positioned in the beginning of the sentence. Therefore, we are looking for a subordinating conjunction that makes sense in the sentence. “Although” fits the bill, since this sentence seems to be contrasting the commonality of blood moons with a reason we don’t see them very often.