Blank Verse is a poetic form which doesn't rhyme but instead a rhythmic pattern marks it out.
Blank Verse is written in Iambic Pentameter meaning that each line has 10 syllables with a stress on syllables 2,4,6,8 and 10.
Iambic Pentameter is most famous in Shakespeare's sonnets:
Shall I compare thee to a summer's day?
It has also been said that it is most like the way we speak with the iambic often also compared to a heartbeat. There are many everyday phrases that fit into this pattern.
Does anybody want a cup of tea?
There are no fixed stanza lengths Blank Verse just relies on the rhythm being maintained.