Blank verses is any unrhymed verses composed in one meter, usually iambic pentameter. The meter is what makes it different than free verse.
Christopher Marlowe was the first English writer to use it and it was referred to as his "mighty line". He is responsible for popularizing it and why Shakespeare decided to use it as well.
Here is an example of one of Marlowe's best work using blank verse, you can see the richness in the text:Was this the face that launched a thousand ships,
And burnt the topless towers of Ilium?
Sweet Helen, make me immortal with a kiss:
Her lips suck forth my soul, see where it flies!
Come Helen, come, give me my soul again.
Here will I dwell, for heaven be in these lips,
And all is dross that is not Helena!
I will be Paris, and for love of thee,
Instead of Troy shall Wittenberg be sacked;
And I will combat with weak Menelaus,
And wear thy colors on my plumed crest:
Yeah, I will wound Achilles on the heel,
And then return to Helen for a kiss.
O thou art fairer than the evening air,
Clad in the beauty of a thousand stars,
Brighter art thou . . .
Faustus, Scene 13, ll. 80-95