This is the tritone interval, also known as the flatted fifth or, colloquially, the "devil's interval". This interval has a breadth of six semitones.
The tritone is widely considered to be the most unpleasant-sounding and dissonant of the Western musical intervals, and, by Western musical rules, cannot resolve. This dissonance is observable in the oscilloscope in the amount of apparent noise in the blue wave.
Because of its dissonance, the tritone interval is not used in church music, and was actually banned by the Catholic church during the Dark Ages.
The Locrian mode, considered to be the least useful of the Western musical modes, is built around the tritone, in the same way that the Major mode is built around the perfect fifth.
The tritone is used at the beginning of the song "Maria" from the musical "West Side Story".
