well, about the river of blood and the boulders, I see tired men, leading the cows to the byres, These men must be very tired after the long day they had, so they are walking slowly, heavy, their bodies are strong and powerful like rocks; however, it takes an effort to carry the pails with the milk, so their blood is rushing inside them like a wild river[I also image they are sweating]. I find this association very effective..
The river of blood, can actually be a river, but he's personifying this river into the lives of himself and Frieda (his daughter with Sylvia Plath) , and the dark past they've since had to overcome (Sylvia's suicide) . So he's saying how dangerous it is, with boulders and blood. But then he says it's balancing unspilled milk, saying that for this moment, this peaceful moment in the country, life is balanced, it's calm.
Their bond (which must be strong after withstanding everything the two have gone through) is so great, and the moment that they're in is so beautiful, that even the moon stood back in awe.