Blank verse is a style of lyrical poetry that is defined as having a specific meter but no rhyme. Most often, poetry written in blank verse is written in iambic pentameter. Frost's "Stopping by Woods" certainly has a very specific meter (iambic tetrameter) but cannot be considered true blank verse because it also has a rhyme scheme.
The four quatrains (four-line stanzas) follow this rhyming pattern: aaba, bbcb, ccdc, dddd. It has been argued that it is the meter rather than the rhyme which causes this poem to resonate with listeners and makes the poem most memorable, which is likely why it is often mistakenly considered blank verse at first glance.