A longer answer, which may help for future reference.
"Passive voice" applies to clauses, not sentences. If you have a finite verb, you have a clause. Here, we have two finite verbs, and therefore two clauses. Let's put them in brackets:
[it is known] [that the sky is blue]
be + past participle is a clue of the "passive voice," meaning that "is known" tells you that the first clause is passive. And this is a passive clause without an "agent," the "someone" who knows that they sky is blue. Who is actually the "agent"? Well, we don't know, but "everyone" is a good bet, though it could be just about anyone (scientists, children, physicists, etc.). If you know the "agent" then you can transform the passive clause into its active form, using the appropriate verb:
It is known (by everyone/children/physicists) that the sky is blue
Everyone knows that the sky is blue
Children/physicists know that the sky is blue.
The clause [that the sky is blue] is "active" and can't be turned "passive" because "is" is not a transitive verb. It's an auxiliary verb.