Blank verse is generally defined as unrhymed verse written in iambic pentameter. In fact, blank verse is not limited to unrhymed verse written in iambic pentameter but can be unrhymed verse written in any meter.
Edward II was Christopher Marlowe's fifth play, written about 1592, coming after Dido, Queen of Carthage (c. 1586), Tamburlaine parts 1 and 2 (c.1587-88), The Jew of Malta (c. 1589), and Doctor Faustus (c. 1589). All of these plays are written primarily in blank verse, in unrhymed iambic pentameter.
Although Christopher Marlowe wasn't the first English dramatist to write his plays in blank verse—that honor goes to Thomas Norton and Thomas Sackville, who wrote Gorboduc in 1561—Marlowe was the first dramatist to break from the stiff, formal, precise (and boring) line of da-DUM da-DUM da-Dum da-DUM da-DUM, endlessly repeated, of earlier plays.
Marlowe varied the interior rhythms of his verse and brought the poetry more in line with the natural flow and rhythms of everyday speech. Marlowe's verse was still clearly poetry—nobody really talks like that—but his poetry was more nuanced and expressive, and carried considerably more dramatic impact. For these reasons, too, the characters in Marlowe's plays had greater emotional range and depth, and the audience could empathize with the characters to a much greater extent.
Because it closely resembled colloquial speech, Marlowe's verse was also more accessible to the audience. It invited the audience to listen to words and appreciate the poetry and lyricism of each line, which helped the audience to better understand the play as a whole.