That's odd. I don't think it's ever been blue. The color you're referring to is not the preview color, but the palette that is applied to the binary image. After you insert the threshold step, it uses a binary palette, which displays pixels of value 1 in red.
You can change the color palette by clicking the edit color palette button next to the palette preview in the toolbar, and assign blue to grayscale value 1. The color blue is the binary palette is assigned to pixels of grayscale value 3. That's why I am surprised that you say it's been blue in the past. The binary image returned by the threshold step is made of pixels values of 0 and 1s. It's always been displayed with a binary palette which uses red for 1.
Hope this helps.
Christophe
