Evidently, a ban on blood sausage in Roman law had been attributed to Constantine the Great. The Hypotyposeis blog has tracked this down and says it wasn't Constantine I, but Leo VI the Wise (ruled 886-912).

"We hereby forbid all persons either to use, or to sell it; and We give notice that, if anyone should, in contempt of the Divine Law, he found to have prepared blood as food, whether he sells it or buys it, he shall have all his property confiscated, and, after having been severely scourged, and disgracefully shaved, shall be exiled for life.

Constantine the Great