The bug

Minecraft appears to attempt to create a smooth transition between two biomes with differing colors by varying the color used on a single block and having it fade smoothly, rather than assigning each block a separate constant color (think smooth lighting on vs. off). However, this ultimately fails, as the mapping seems to be completely reversed from what it should be, resulting in sharp borders existing.

How to reproduce

1. Go to the border between a swamp and another biome
2. Place down a large platform of a white block to aid visibility
3. Flood the top of this platform with water
4. Set Biome Blend to 3x3

Expected results

Despite the low biome blend setting, the transition between the biomes would still be smooth with no harsh borders, since this is probably what the game attempts to do. Or at least, any such borders wouldn't be as harsh as they are currently.

Actual results

The gradient, while visible, goes the wrong way, accentuating the edges between blocks.