The witching hour can be dismissed with one question: Which 3am is the correct 3am? England? Scottland? U.S.A East or west coast? etc. The world runs on a 24 hour clock, so which time is the right time for the witching hour?
This does not fit the definition of an oxymoron.
Your point can be dismissed by simply pointing out that blasphemy, which is the driving force behind this, does not depend on absolute time. Blasphemy is symbolic. It simply does not matter which 3am we are talking about. IOW, a blasphemer can blaspheme at 3am 24 times a day.