One material becomes dark/black after baking tried to bake all my house materials into one. But for some reason my roof texture ends up being black after the diffuse bake. I'm quite new to this stuff, but I'm sure I did it as the baking tutorial instructed and everything else seems alright. The original texture is greyish just like it looks in the original (left) object.
EDIT: Actually now that I compare the models more, everything seems to be a bit darker than the original, but I'm guessing it and what's happening to the roof are two separate things, so I'll wait for the answer and ask about the second thing later.
So, I found the answer at last. Here's for you that have the same problem:
I'm not quite sure why but if you have a full metallic material (perhaps it darkens less if your material is less metallic. Didn't have the time to play with the bake), the diffuse baking will bake the material as black. So to fix this issue disconnect the metallic map/put the metallic value to zero before baking diffuse and voilà the diffuse map will have the real color.