What Revit wants is for you to increase the scale or spacing between the individual lines of the Filled Region pattern - then it will switch back to black, as it should be.
Using a Model pattern, I was able to observe the pattern go from light gray to black as I changed the scale (spacing) between the lines.
Interestingly, using a Drafting pattern allowed me to put the lines in the Filled Region much closer together...
Some more info at:
RevitCity.com | filled region pattern color changes when drawing scale changes