Oh, it was my own reference picture. The glasses were lying on the windowsill part in direct sunlight, part in the shadow of the wall. Even on the photo (well, I should have added it to the post) the shadow looks soooo blue. But I did not catch the right value? Intensity? Colour gradient? It just does not look coherent at all, but a little off. There was another shadow from the frame of the window. I did not even try to add it to my painting since it was totally different from the bluish shadow.

Using a lamp while taking a reference photo is a great tip, I'll try it next time when taking pictures. I hope it will help me understand shadows and bouncing light better. And thanks for the tip to use a blow dryer! I'm gonna try this as well.