Well DV 5/1 compression is our curse but renderings shouldn't gives you any blocking... check your settings.
Magic Bullet has this very good deartifacting plugin, renders are slow as death but you get good results.
A tip which is a rip of a DV chromakeying technique is selecting the most blocky channel (most often the Blue or Green one in DV) and apply a subtil(2-3 pix max) directionnal Blur or Median effect.
