The first part is easy. Even among alchemists, blasphemy to God... と is tacitly forbidden, I hear.
What I am thinking is that it might be a quotation. As in 「神への冒涜」と is being forbidden. と in this case would specify what is tacitly forbidden.
Is it just that or does it have another meaning that I am missing.
暗黙 means tacit and the のうちに makes it tacitly. But usually (の)うちに means while; during; as when coupled with a noun. So is 暗黙のうちに just a common expression or is there more to うちに that meets the eye.