Blue flower on a red hydrangea bush

Blue flower on a red hydrangea bush, originally uploaded by Rhian vK.

In my neighbours' garden.
In fact, it's not all that surprising, as I'm told that this bush grew from
a cutting from an established plant round the corner, by their garage, and
that that one has blue flowers.
But that is what is surprising!
You wouldn't imagine that hydrangeas and their cuttings, growing close by
and on similar soil, should produce flowers of a different colour.