The Preferences menu contains a Local Undo button that applies to the Timewarp Editor, Colour Correction Editor, and Blend Editor. When Local Undo is enabled, all events in these editors are added to the undo buffer in a linear sequence.

When Local Undo is off, all events in these editors are grouped into a single event once you exit the editor. Thus, all those events are “undone” with a single use of Undo.

After exiting the editor, you can turn Local Undo on or off to change the state of the undo buffer for events that are already in the buffer (that is, turning the Local Undo button on or off affects the items currently in the undo buffer, not just the ones added after you turn it on or off).