It looks like your sage has been attacked by sucking insects which produce a honeydew which in turn grows a sooty mold. Sooty mold is a fungus which grows on the honeydew. As the name implies, it is sooty in color and the honeydew can be produced so heavily that it drops from the leaves to objects below the trees, where the fungus can grow and look like pitch.

Control of the insects would be necessary to prevent sooty mold. Control of sooty mold is targeted at preventing or reducing insect populations. Scales and aphids are usually the culprits in sooty mold infestations. It's important to identify the species of insect present by checking your plants and see if you see aphids or scale. After finding which insect is doing the damage, you can then determine what control measures to apply and when. Check your plants and see if you see aphids or scale.

The following sites have more information on both aphids and scales:

http://www.na.fs.fed.us/spfo/pubs/howtos/ht_sooty/ht_sooty.htm

http://learningstore.uwex.edu/assets/pdfs/A3123.pdf

http://msue.anr.msu.edu/news/using_horticultural_oils_on_landscape_plants

Thanks for your inquiry, and good luck!