In the new Doctor Who series, we are frequently reminded that the Doctor "becomes a part of the events" as soon as he enters a time period, and is unable to (or should not) backtrack to fix his mistakes along that series of events.
The question is (please refer to the diagram):
While he cannot travel back along a series of events of which he is a part (i.e. within the blue rectangle), what prevents him from traveling further into the past beyond those events and leaving a message for himself, or better yet, geting a proxy to help him out? Isn't this what he did in Blink? Why didn't the reapers appear then? How do we know when it's just "wibbly wobbly timey wimey" stuff and when it's time for the reapers?
Edit/Clarification: In Blink, a future Doctor stuck in the past forwards information to the present to get someone to alter his timeline. No more than one copy of the Doctor was present at any given point of the timeline, but the net effect was the same -- alteration. Why was this allowed?