I don't know why, but the hearth has this visual problem even when this is the only mod loaded. It appears only in the latest version (1.3.4). The Forge version is 36.2.0

Thank you for bringing this to my attention. I was able to solve the problem of the hearth appearing black in a test world and will release an update to Cold Sweat soon. I'll be closing this post soon after the release if the bug is resolved.
Great bug report, by the way! It was very helpful to be able to see pictures and a video.