They were a transition. The black pants were introduced in 2006 but were considered "foul weather" gear originally. They still wore the solid knickers also. The NFL officials became enamored with the black pants and so some referees started declaring "foul weather" when there really wasn't any as I recall because they liked the black pants better.
The black stripe on the knickers was added in 2008-2011. In 2012 they wore only black pants.
The Gridiron Uniform Database details all teams AND officials uniforms.
http://www.gridiron-uniforms.com/GUD/officials.shtml