I've been puzzling over the old maps. The grid reference given is where the stone's marked even now. I was excited to find this photo on Geograph - doesn't it match the description well? But perhaps that's what rocks look like round there - I think it's not quite on the spot where the grid reference is. So that's confusing. We need an on-the-spot reporter.
Though I'm not sure it's worth the risk of finding out if the rumours are true. Or maybe it is. Might be untrue, and if it is true, you've got a 50:50 chance.
In a stony place, called Yr Arddu, Black Ham, pretty high in Cwm brwynog farm, on the ascent of Snowdon hill, there is a very large loose stone, called Maen du yr Arddu, i.e. The black Stone of Arddu; upon the top of which there is another lesser stone, seemingly as if it had been raised there by hands.
It is said, that if two persons were to sleep a night on the top of this stone, in the morning one would find himself endued with the gift of poetry, and the other would become insane.
And accordingly it is affirmed, that in a frolic two men, one called Huwcyn Sion y Canu, and the other Huw Belissa, agreed to sleep on the top of it one summer night: in the morning one found himself inspired with the celestial muse, and the other was quite bereaved of his senses.
It seems that both of these were of the lower order of minstrels, and very probably both of them drunk when they slept there: one, it should seem (having the appellation y Canu, Singer or Songster added to his name, and being addicted to singing) found his spirits in the morning in an exhilerated state, and the other not quite recovered from his intoxication. Imagination might have co-operated, so as to make him who was cheerful to fancy that he was really inspired, and to give the other an idea that he was really mad.
Or: how to kill a romantic idea stone cold dead with the application of reason.