Quick Illustrator tip…
When using gradient opacity masks, the mask will not go fully opaque unless the black swatch of your gradient is in RGB mode.
Default for gradients is Grayscale mode:
Illustrator Opacity Mask Gradient Grayscale
To fix this, double-click on the black swatch, then click the dropdown in the top right of the box that pops up. Change the mode to RGB.
Illustrator Opacity Mask Gradient RGB
If you’re not sure where to start with opacity masks in Illustrator, Bittbox is a great place to learn the basics.