Stonehenge

Find it hard to see them as blue! Though I am sure they came to Stonehenge for a special reason, the locals must have had good relations with South Wales. to acquire the stones.

Of course not. :)

Interestingly, some have traces of iron and copper, which might give them unusual properties.

However, there's a pretty strong case for them being transported by glaciation to Salisbury Plain, and not being valued particularly in their native region, or elsewhere in Britain.

http://www.britac.ac.uk/pubs/proc/files/77p133.pdf
http://onlinelibrary.wiley.com/doi/10.1111/j.1740-9713.2008.00284.x/full

Bluestone was valued back in Wales. There are quite a few ritual axes made of Bluestone and several Megalithic monuments back in Wales use Bluestone in their construction e.g. Pentre Ifan Dolmen, Waun Mawn Standing Stone and several of the stones at Gors Fawr Stone Circle. Bluestone is also credited with healing properties in Welsh folklore, especially when water is run over the stones - rather like in Monmouth's story.
The reason they are called Bluestones is that the Welsh word 'Glas' which is often translated as 'Blue' technically means 'green/grey/blue'.