Is the impurity of blood transferred from an impure surface to a pure surface only when blood is seen on the pure surface?

A pure place is not deemed impure merely by doubting the transference of impurity to it. The fundamental principle regarding things is purity, and impurity is an incidental matter that is not transferred except with certainty, due to the two jurisprudential rules: "The default is that what was, remains as it was, until proof to the contrary is established" and "What is established with certainty is not removed by doubt."