On second image you have problem with your texture, it looks like it is missing. Please reload image in to this channel and see what will happen? I think that is reason why on the third image is black your material preview window .
@danbush - I think 3cm is to large. This is only a visually effect, not a change in geometry. If you want large rounded edges, you'll need to model it. Try 0,5cm (or lower) and see how it works.Hope it helps