How do I change the color created when erasing?

[BlueFrog]

Hi,

I am trying to make a tab for a menu: I have a black rectangle on transparent background and use the eraser to round off the top corners.

However, the eraser creates very light gray pixels, and the background of my menu is dark gray.

How do I change it so the eraser creates dark gray pixels, not white / light gray?

Thanks!

[LucasZ21]

convert your background color to dark gray?

[BlueFrog]

Yes, I did that, but still it would make the fading parts lighter that my background color...