I can give some advise perhaps. The black is likely a virus that has infected the plant long ago. Everyone I know that has a Salvia plant in an arid environment experiences this blackening.
The thing that has helped my plant the most has been the summer humidity. It actually grow like a normal plant...almost. You could still see that the leaves were very dry though.
This plant needs a humidity chamber unless you are growing it in a tropical environment. I plan on building a humidity chamber for it and my p. viridis plant this winter.
Sorry I can't be more help. Salvia is a very special plant and the genetics are not diverse at all. If you can manage to get that plant to produce seeds many people would be very interested in those seeds.
Good Luck!
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