Black brush shows milky eefect

[blogsofwardotme]

Hi, I have been working with the brush for several weeks in 3.0.2 with no problem. I am working in the layer mask to the right of a copied selection, as in the screenshot.
I am working with the black brush in the white part of the image, to remove the last traces of white which surround the pasted element in the second layer. So, the black brush should bring out the dark area which is in the original image under the mask .
If I work on the dark area of the image, nothing should change. If I run it on the white left over from  the mask (circled in the screenshot), it should expose the dark underneath. So it did for days.
Now the brush is milky as in the screenshot. My opacity is 100 both in the brush options and layers panel, color picker solid black.
I don't know what I touched, thinking of going to color gradient, what would be the default? Closed Gimp multiple times, redownloaded Gimp. Thank you.

[rich2005]

(9 hours ago)blogsofwardotme Wrote: .....snip.....
I don't know what I touched, thinking of going to color gradient, what would be the default? Closed Gimp multiple times, redownloaded Gimp.  

Reinstalling Gimp is rarely any use, by default your settings are retained in your Gimp User Profile which remains untouched between installs. It also remains between close-and-restart because the default is Save tool options on exit  and of course those settings are used when you restart.

If, anytime, you are really stuck and do not know what you changed. In Preferences -> Tool Options click on Reset Saved Tool Options to Default Values  then OK and restart Gimp

Looking at your screenshot, my comments.
Nothing wrong with making your own brush, I use similar, small, more fuzzy than standard. Can be useful in a layer mask to "blend in" an edge. Up to you what you use.
Using a Dynamic (1) Color from Gradient - Black to White Also a small Fade 13 ? and a Triangular Repeat.
That will paint as a repeating pattern Black-to-White as (2)
Why would you want to use that in a layer mask (3) ? It will hide / show with some semi transparency as shown.