All types of mpeg compression can suffer from blocking artifacts. That includes h.264. The problem occurs because mpeg compression works with blocks of pixels and quantifies them to a single value. The blocks are small, but, on decoding, adjacent blocks that are the same or very close value can appear to be part of much larger blocks and you get that blocky appearance. It's more likely you will get blocking in areas with low contrast and/or limited dynamic range, but you can get it in high contrast areas as well. You can also get blocking from the nature of predictive intermediate frames in mpeg compression. This shows up especially in areas with motion. The blocks can appear large, blotchy or grimy, or even jump around.
If the video contains either very high contrast or very low contrast images or subtle gradients, you might get better results if you enable the Render at Maximum bit-depth option. This will use 32bit color calculations and potentially result in better quantization. The trade off is increased rendering time. If you are getting predictive motion blocking, reducing the mpeg maximum keyframe distance may help.
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Since you have a higher bitrate, you probably are at level 4.1 or higher already. But, double check the profile setting. "High" is designed for HD content, "Main" is designed for SD content. The Profile setting constrains the compression algorithms used and the number of macroblocks among other things. You should be at "High". I've noticed that with Premiere Pro CS6, the Profile setting is sometimes changed to Main when I make other adjustments in export and I have to manually go back in and change back to High.
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