From: Rabbi Moshe Sofer
To: Rabbi Yaakov Ehrlich
The Chasam Sofer rules that the clear liquid (tzelalasa d'dama) seeping from congealed slaughter blood is forbidden and renders a cooked dish prohibited. While there is no kares liability, the prohibition stands as a Biblical lav according to Rashi and most Poskim (as dam ha'eivarim), and is at minimum Rabbinically forbidden according to Tosfos. Independently, even if the liquid were no longer classified as blood, it remains forbidden as tzir (extract) from a prohibited substance under the principle that liquids emerging from prohibited items share their status.
Source: Shu"t Chasam Sofer YD §70