In the film, I think it's for dramatic effect. The dark Mark, the sign of evil and, for many, cowardice, against Harry's bravery. In this scene, blood doesn't carry negative connotations as it often does but rather reinforces the fact that Harry still has blood in his veins and has once again survived. The juxtaposition of the two "marks" is a powerful image. It is also, perhaps, a hint at the link between Harry and Voldemort's respecive survivals- a hint at what is to be revealed in later instalments. However, as the comparison doesn't happen in the books I don't think it's significant in terms of plot or explanation.