Did you ever resolve this issue? You may want to get a soil sample test kit from your garden center and do a soil test to see what the ph balance in your soil is.

Are you having a similar problem with other plants? I don't live in your garden zone - I'm north in a totally different climate, zone 5a. I grow anise hyssop like there is no tomorrow. I have a zillion seeds and could send you some (they start easy from seeds in a tray). Just email me your address through My Page and I'll pop some in the post. No guarantees they will get there because customs might nab them - I can send them inside a garden card.

I've never heard of anise hyssop turning black before. In a google search I've found anise hyssop will turn black if it gets frost, serious cold.

Cheers,
Peggy