Other mistake: In the original "Scooby-Doo" episode from which this episode is taken from, the house for the reading of the will was on an island as the Mystery, Inc. gang travelled to the house via motorboat. In this episode of "Supernatural", Sam, Dean, and the Mystery, Inc. Crew merely drive up to the house.
Continuity mistake: After Dean zaps the TV with the remote, he lowers his right arm. The next shot behind Dean's right side shows him lowering his arm again.
Answer: Angel blades come from angels. It's their angelic magic that makes it special. Therefore, an archangel blade is made magical with the powers of an archangel. They couldn't have just created one or used one, as none of them are archangels.