In a sense, this is comparing apples with oranges. Blake's "A Poison Tree" is a short lyrical poem, and Romeo and Juliet is a play. One was written in the late eighteenth century and the other over two centuries earlier. While Shakespeare was probably a fairly traditional Christian, Blake in this as in his other poems was concerned with inventing his own religion. Shakespeare's drama is about two young lovers, and Blake's poem focused on the nature of all human relationships, not love per se.
Although you could make arguments about using Blake's approach to the nature of feuds in general to the feuding families in Shakespeare's play, the comparison seems rather random. It is usually better to start with a thematic point of comparison and then choose examples than to choose two works at random to compare. For Blake, a better comparison would be Milton, as Blake was fascinated by Milton; you could compare "The Poison Tree" with Milton's account of Lucifer's fall. For Romeo and Juliet, you might want to compare it to other stories in which families oppose the wishes of two young lovers or stories of family feuds; As You Like It or The Merchant of Venice might be good choices.