Adjusting Color Saturation, Hue, and Tone?

I'm using GIMP right now to adjust my digital photos. When I adjust the hue, saturation or tone of a certain color in an image, any similar color will become pixelated. For instance, when I darken a blue hue, any lighter blue value will dramatically stand out and look very cubed, or pixelated.

Is there anyway to make these adjustments without creating this negative effect?

Someone mentioned working with masks/quick masks might remedy the problem. But, I'm not sure what they meant. I'm not too familiar with using masks, so how would this be used in my situation? For that matter, what are layer masks?

Thanks.