I don't know that I would call this a "joke" of any kind, but I suppose you are really asking about Owen's deliberate choice of words here being at once a metaphor and, almost, a homophone for something else. By "blood-shod," Owen means to imply that the soldiers are so weary and struggling so much, having walked so far for so long, that they are wearing blood on their feet rather than shoes. The soldiers in question, the "some," have lost their boots in the struggle of the war but are unable to do anything other than continue to march, leading their feet to become torn, shredded, and bloody. They are not literally "shod" in blood—indeed, they are unshod—but their feet appear red with blood. Owen plays a lot with sound and rhymes/half-rhymes in his poetry, too, however, so we can also pick up on the similarity of sound between "blood-shod" and "bloodshot," a much more commonly used word. "Bloodshot" usually refers to the way our eyes appear when we are sore and very tired, so it seems to fit in this context. We can imagine that these very weary and tired soldiers are likely to have bloodshot eyes to accompany their "blood-shod" feet, completing the picture of exhaustion Owen paints.