The source of Blackmoor special magic

[Havard]

Right, but the stone itself is magical for some special reason or it's just by sheer "luck"?
According to most sources, Blackballs tend to slip through [existing] tears in the fabric of reality, rather than producing the tears by themselves...

The difference between Rob's theory and mine is that the Blackball in my version is more a symptom than the cause. It is the presence of the Old One (temporarily) that infuses the rock with magic. The Blackball is simply a guardian he left behind.

I do like Rob's theory of the ley lines connected to this though. Perhaps there are Blackballs at the end of each ley line?

-Havard