The Catoctin Mountains are not actually part of the Blue Ridge. The Blue Ridge is the western half of an eroded anticline and the Catoctin Mountains are part of the eastern half (along with the Bull Run Mountains, the Pantops, Monticello, Fan Mountain, and others in VA).
According to the US Park Service "Catoctin Mountain Park lies within the mountainous area known as the Blue Ridge Province."
Most literature also agrees with this.
The coresponding areas in Virginia are considered piedmont rather than part of the Blue Ridge. South Mountain is the ridge in Maryland that is the continuation of the Blue Ridge in Virginia.