The early black stones began on my home stream in Pa in late Feb or early March. I have never seen a more intense and long lasting black stone hatch, or emergance. The best dry we used was a well hackled, black ehc styled fly. The black stones are available to trout in big numbers when the females slam onto the surface in big numbers to lay their eggs. Very often they skitter accross the surface and the well hackled fly was great at imatating this. Many years our biggest brown was caught in March during the black stone fly hatch.
Oh yeah, I liked to put an underwing of a greyish wing material under the top wing. When those flies hit the surface their wings are fluttering.