Why would a blended salad dressing change its color?
It seems I was looking in the wrong direction here. It seems to have been the Mozzarella cheese. After pouring the dressing over the salad, the cheese started to leak some white liquid, which changed the colour of the dressing.

This was something I hadn't on my radar, since I had always used the same brand of cheese, and it had never happened before. I only found out because last week I did a shaken dressing, and this caused that behavior too. So it's nothing to do with the blender ...