There's a lot of "doctor/companion does X, and Y happens" moments in DW, but some of the time it's more of a chicken-and-egg thing, such as the case with Blink. The Doctor knows what to do in 1969 because he got it from Sally, who got it from the Doctor himself.
... Yeah, I don't really get it either, but hopefully this will explain it a bit better (http://tardis.wikia.com/wiki/Temporal_paradox)
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... Yeah, I don't really get it either, but hopefully this will explain it a bit better (http://tardis.wikia.com/wiki/Temporal_paradox)