I have just read about blank verse and free verse in the glossary of literary and theoretical terms of Studying Literature: The Essential Companion by Paul Goring, Jeremy Hawthorn, and Domhnall Mitchell.
Blank verse in English literature traditionally consists of a sequence of lines of unrhymed iambic pentameter; Antonio’s melancholic speech in the opening of Shakespeare’s The Merchant of Venice is a good example.
In sooth, I know not why I am so sad;
It wearies me; you say it wearies you;
But how I caught it, found it, or came by it,
What stuff ’tis made of, whereof it is born,
I am to learn;
And such a want-wit sadness makes of me
That I have much ado to know myself.
(A bit of aberration in the fifth line perhaps – or maybe the speak is supposed to pause for three metrical feet before continuing.) Wordsworth’s autobiographical poem The Prelude provides another example of blank verse.
Free verse is simply poetry without any formal metrical structure or fixed line length; it may rhyme but need not.