Ohappydayes: Thank you for your suggestion of what it might be. Unfortunately it can't be brushes, as I use a flocked sponge. I thought it might be brushes too, so I changed to a flocked sponge and it just got worse. I wash the flocked sponge everyday.
Zarine: Thank you for that suggestion, as well. I will try it.
Autumnagain: I haven't changed my skincare routine at all, so I haven't figured it out.
Ruby2: I was considering Mineral Boutique, but I was concerned about the color match. Looking at the available colors, the darkest color doesn't look like it would be a match for me. If you're familiar, my coloring is somewhere around Angela Bassett and Whitney Houston.