Both lines can be considered correct. When John McCrae first penned the poem in the spring of 1915 he used "grow". When the poem was published in Punch Magazine in December of 1915 an editor (with McCrae’s permission) changed the line to read "blow" noting that "grow" was repeated in the last stanza of the poem. McCrae himself interchanged the two words in the poem over the next few years before his death in 1918.