The title is meant to be ironic; the man is anything but “A Blessing” to the girls inside the barbed wire. The poem as a whole is meant to display a façade, all to mislead readers and lead them away from the stories meaning.
To conclude, viewed through the eyes of a mentally deranged person all the barriers we as a society put up inevitability to protect ourselves can be crossed as easily as a man walking over barbed wire. We are never really safe, and that is the illusion.