"Darkened" is the past participle of the word "dark," a participle being the "ed" or "ing" form of a verb. In the example you give, "darkened" functions as an adjective and indicates the tense of the word "dark." This shows how it functions in time: the room used to be light but, over a period of time, became dark. In the second example, the room is simply characterized as dark. "Dark" in that sense is used simply as an adjective to describe the room.