When leaves on lavender turn black and it is not due to lack of drainage or overwatering I suspect lavender shab, a fungal disease caused by spores that love to infest the older heirloom lavender varieties. To save your plants work quickly to remove every infected leaf and put into a bag not the compost. Collect the fallen leaves as well, as spores will fall to ground and reinfect plants. You may want to use a fresh mulch to help seal in any of the fallen spores. The newer lavender varieties are more resistant to shab disease. Good air circulation and not getting the foliage wet also helps. You may want to take cuttings form the healthy tip growth and uproot and destroy the infected plants. Keep growing, Marianne Binetti