Lots of us are looking for him, and no-one has found him yet. I actually think it was a mis-hearing of Westerham. The Sloper family seem to have arrived in Eltham at about the same time as the Blackstones, and they originate near there. William Sloper married William and Elizabeth's daughter Sarah, and took on her illegitimate son Thomas, so I get the feeling the families were close friends.
There were very few Blackstones in the West of England at that time, but quite a big bunch of them in East Grinstead, and William seems more likely to have wandered north from there, but I can't find evidence for it.