G. D. Griffiths

British author. Full name Gordon Douglas Griffiths. As well as one hard to find Dartmoor-based pony book he wrote another story set on Dartmoor about an abandoned kitten (
Abandoned) and a story about a hedgehog (Mattie). He also, I think, wrote a history of Teignmouth in Devon.

Horse & Pony Books:

(1st edition WORLD'S WORK 1970)
SUMMARY: The story of a strain of silver blue coloured wild ponies set on Dartmoor

Collectors info:
I don't think Silver Blue was ever reprinted and is therefore fairly rare, although there are usually one or two copies around, in the UK at least. Price can vary a lot. The author's other animal stories are however much easier to find.