MPEG2 only stores colour information once every 2 pixels, since the human eye is more sensitive to differences in luminance (brightness). Deep reds have low luminance as well, which makes them look worse. Having said that, the blockiness I've seen on some R1 discs (The Abyss, for example) looks far worse than can be accounted for by the chroma subsampling (the every-2-pixels thing I was talking about). My R2 Abyss doesn't show these artefacts.