If it’s happening with a multiple on the bump then that’s probably it. To high or to low can cause black especially if you have a strength of 1 on the bump node. Also make sure you have your bump map texture plugged into the height of the bump node. You can also try adjusting the strength. Transparency will go black if you don’t have enough bounces for transmission(I think that’s what the bounce setting is called, it’s been awhile. I rarely go over the default)
