a bolt of lightning right off the nose, from an apparently clear blue sky
Yes, I once saw a bolt of lightning out of an apparently clear blue sky (ok so it was a bit murky, but nothing out of the ordinary and there were no visible clouds as such). A few minutes later the "clear blue-ish sky" seemed to have turned into a line of unforecast CBs.
I couldn't get past them, and had to land somewhere to wait for them to go over.