You are correct in saying that "bluestone" isn't a specific geological rock type. The Stonehenge ones appear to be a type of dolerite, an igneous rock which in turn has several types. In Cornwall we have a hard dolerite that we call greenstone, which forms coastal headlands that protrude out to sea beyond the areas of softer slaty rocks and provide spectacular cliff scenery (e.g. at Pentire Head, at the Rumps peninsula, my favourite spot on Planet Earth and where my ashes will be scattered). I'm not entirely convinced by the glacial erratics theory. Salisbury Plain and the stretch between there and Pembrokeshire was at the extreme southern edge of the maximum glaciation and I have to wonder whether the ice had enough consistent heft to get those rocks all the way, and whether the ice would have flowed in the right direction in any case. I wonder whether any drop-offs have been found along the route, which is what happens with large erratics (I know the Norber Boulders in Yorkshire)... Musing from ignorance here...