The issue you are having is that when you precomp your adjustment layer you are taking away what you want to adjust. This is assuming that you made sure move all attributes into the new comp when you made the precomp. What you would need to do to allow it still adjust whats below it is turn on collapse transformations. This however would not solve your problem because the blue will not give you full opacity.
What you can do is in your precomp make a solid and make a ramp from black to white. Then create a blue solid in your main comp and set that solid to luma matte and it will give you a gradient from blue to transparent. There are are other ways to do this with out precomping like using the brush tool set to paint alpha but this way seems the simplest to me and give you more flexibility overall I think.
The Luma Matte picks up luminance, sorta similar to brightness to create areas of transparency. If your blue is dark, then you'll get very little transparency after using it as a color within your track matte.
If all you want is a gradient, from blue to transparent, the apply the Lighten Blending Mode to the gradient layer.
Another option is to apply Shift Channels on the gradient layer and shift one of the RGB Channels into the Alpha Channel. In your case, the Blue will be an ideal choice.