I found 2 mods that fix the issue of the drastic difference between the swamp biome and the neighbouring one.
#1 - Fixed Swamp and Biome colors! Great for texture packs! Minecraft Mod
If you don't want the darker grass and water at all, then this mod is for you (which is what I'm using and works like a charm).... All swamp grass is normal green grass and swamp water is normal water, so there is no change to swamp at all (except the usual mushrooms and vines)
The mod gives no instructions, but you unzip the downloaded file and drag the files that were in the zip into your minecraft.jar (making a backup of your jar just incase something goes wrong)
#2 - [1.0.0] Better Swamps - Minecraft Forum
If you still want the swamp grass/water but want a better blend, then try this one - I've not tried it, as I'm not a fan of the change in water, but it looks good. that thread gives a great example pic of a before and after showing how the mod blends the water and grass, so you get a gradual change at the border of the biomes.